Part 1: Making the Cookie Dough

1. Combine Dry Ingredients: In a large mixing bowl, whisk together the 4 cups of all-purpose flour and 4 teaspoons of baking powder. This ensures the leavening agent is evenly distributed, which is key for a uniform rise. Set aside.
2. Cream Butter and Sugar: In the bowl of a stand mixer fitted with the paddle attachment, or in another large bowl using a hand mixer, cream the softened butter and 1 cup of granulated sugar together. Beat on medium-high speed for 3-4 minutes, until the mixture is light, pale, and fluffy. This process incorporates air, which leads to a tender cookie crumb.
3. Add Eggs and Flavorings: Add the eggs one at a time, beating well after each addition and scraping down the sides of the bowl as needed. Once the eggs are fully incorporated, beat in the vanilla extract and fresh lemon zest.
4. Alternate Wet and Dry Ingredients: With the mixer on low speed, begin adding the flour mixture and the milk. Add the flour in three additions, alternating with the milk in two additions (Flour, Milk, Flour, Milk, Flour). Mix only until the dough just comes together. Over-mixing can develop gluten and make the cookies tough.
5. Chill the Dough: The dough will be soft and slightly sticky. This is normal. Turn the dough out onto a lightly floured surface, gently form it into a disc, and wrap it tightly in plastic wrap. Refrigerate for at least 1 hour, or up to 24 hours. Chilling the dough is a critical step; it prevents the cookies from spreading too much during baking and makes the dough much easier to handle.

Part 2: Baking the Peach Halves

1. Preheat and Prepare: Preheat your oven to 350°F (175°C). Line two large baking sheets with parchment paper.
2. Portion the Dough: Remove the chilled dough from the refrigerator. Using a small cookie scoop or a tablespoon, portion the dough into small, uniform balls, about 1 tablespoon (or 20-25 grams) each. Roll each portion gently between your palms to create a smooth, round ball. Uniformity is key to creating perfectly matched cookie pairs later.
3. Arrange and Bake: Place the dough balls on the prepared baking sheets, leaving about 2 inches of space between them to allow for some spreading.
4. Bake to Perfection: Bake for 12-15 minutes, or until the bottoms are a light golden brown and the tops are pale and set. They should not be browned on top. The texture we’re aiming for is more cake-like than crispy.
5. Cool Slightly: Let the cookie halves cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool a bit more. They should still be warm for the next step.

Part 3: Preparing the Luscious Pastry Cream Filling

You can prepare the crema pasticcera while the cookie dough is chilling or while the cookies are baking.

1. Infuse the Milk: Pour the 2 cups of milk into a medium saucepan. If using a vanilla bean, scrape the seeds into the milk and add the pod as well. Bring the milk to a gentle simmer over medium heat. Do not let it boil. Once it simmers, remove it from the heat and let it steep for 10-15 minutes to infuse the vanilla flavor.
2. Whisk Yolks and Sugar: In a separate medium bowl, whisk the egg yolks and 1/2 cup of sugar together until the mixture is pale yellow and has thickened slightly, about 2 minutes. Whisk in the 1/4 cup of cornstarch until no lumps remain.
3. Temper the Eggs: Remove the vanilla pod from the warm milk. While whisking the egg yolk mixture continuously, slowly pour about half of the hot milk into the yolks. This process, called tempering, gently raises the temperature of the eggs without scrambling them.
4. Cook the Custard: Pour the tempered egg mixture back into the saucepan with the remaining milk. Return the saucepan to medium heat and cook, whisking constantly and vigorously, until the cream thickens. This will happen quickly. Pay special attention to the corners of the pot. Once it’s thick, continue to cook while whisking for another minute to cook out the raw starch flavor.
5. Finish the Cream: Remove the pastry cream from the heat. Immediately whisk in the cold butter and the optional rum or Grand Marnier until fully incorporated. The butter adds a beautiful shine and rich, silky texture.
6. Chill the Cream: Transfer the finished pastry cream to a clean bowl. Press a piece of plastic wrap directly onto the surface of the cream to prevent a skin from forming. Refrigerate for at least 2 hours, or until completely cold and set.

Part 4: The Art of Assembly

This is where the magic happens and your cookies transform into peaches.

1. Create the Cavity: While the cookie halves are still warm but cool enough to handle, use a small, sharp paring knife or a melon baller to carefully scoop out a small cavity in the flat bottom of each cookie half. Be gentle to avoid cracking the cookie. Save the cookie crumbs you remove—they are a secret ingredient!
2. Enhance the Filling: Take your chilled pastry cream from the refrigerator. Give it a good whisk to loosen it up. Take the cookie crumbs you saved and mix them into the pastry cream. This not only prevents waste but also adds fantastic texture and stability to the filling.
3. Fill the Cookies: Using a small spoon or a piping bag, generously fill the cavity of one cookie half with the pastry cream mixture.
4. Form the Peach: Gently press a second, unfilled cookie half onto the filled one to create a complete sphere, resembling a peach. Some filling may ooze out; you can smooth this with your finger or a small spatula. Place the assembled “peach” on a tray. Repeat with the remaining cookies.
5. Prepare the Dip: In a shallow bowl, mix the Alchermes liqueur with the water or milk. The dilution makes the color a bit softer and the flavor less intense, which is traditional. In a second shallow bowl, spread out the 1 cup of granulated sugar.
6. Dip and Roll: Working with one assembled cookie at a time, quickly dip it into the diluted liqueur, ensuring it’s coated all over but not saturated. Immediately transfer it to the bowl of sugar and roll it around until it’s fully coated. The sugar will cling to the damp surface, creating the classic fuzzy peach skin effect.
7. Final Garnish and Chill: Place the finished peach cookies on a serving platter or in a container. For the most authentic look, use a fresh mint leaf or a piece of a candied cherry to create a “stem” at the top where the two halves meet. Refrigerate the finished cookies for at least 1-2 hours before serving. This allows the liqueur to soften the cookie slightly and the flavors to meld together perfectly.