Part 1: Preparing the Boiled Raisin Base

1. Combine and Boil: In a medium-sized saucepan, combine the 2 cups of raisins, 1 ½ cups of water, ½ cup of cubed unsalted butter, 1 cup of granulated sugar, and ½ cup of light brown sugar.
2. Heat Gently: Place the saucepan over medium heat. Stir occasionally as the butter melts and the sugar dissolves.
3. Simmer: Bring the mixture to a gentle boil. Once it’s boiling, reduce the heat to low and let it simmer for exactly 5 minutes. This step is crucial for plumping the raisins and creating a flavorful, syrupy base for the cake.
4. Cool Completely: Remove the saucepan from the heat. This is the most important step for the success of the cake. You must let this mixture cool down to room temperature. This can take 45 minutes to an hour. If you add the eggs while the mixture is hot, they will scramble. You can speed up the process by placing the saucepan in a cold water bath, but be patient—cool is key!

Part 2: Mixing and Baking the Cake

1. Preheat and Prepare Pan: While the raisin mixture is cooling, preheat your oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan. Alternatively, you can line it with parchment paper, leaving an overhang on the sides to easily lift the cake out later.
2. Combine Dry Ingredients: In a large mixing bowl, whisk together the 2 ½ cups of all-purpose flour, 1 ½ teaspoons of baking soda, 1 teaspoon of cinnamon, and all the other ground spices (nutmeg, allspice, cloves), and the ½ teaspoon of salt. Whisking thoroughly ensures the leavening and spices are evenly distributed, preventing pockets of baking soda or clumps of spice in the final cake.
3. Add Eggs and Vanilla: Once the boiled raisin mixture has cooled completely to room temperature, whisk in the 2 room-temperature eggs one at a time, followed by the 1 teaspoon of vanilla extract. The mixture should look glossy and slightly thickened.
4. Combine Wet and Dry: Pour the cooled, wet raisin mixture into the bowl with the dry ingredients. Using a rubber spatula or wooden spoon, fold the ingredients together until just combined. Be careful not to overmix. A few small lumps in the batter are perfectly fine. Overmixing will develop the gluten in the flour and result in a tough, dense cake.
5. Pour and Spread: Pour the batter into your prepared baking pan and use the spatula to spread it into an even layer, making sure it reaches all the corners.
6. Bake: Place the cake in the preheated oven and bake for 30-40 minutes. The baking time can vary depending on your oven. The cake is done when a wooden skewer or toothpick inserted into the center comes out clean or with a few moist crumbs attached. The top should be golden brown and spring back lightly when touched.
7. Cool the Cake: Remove the cake from the oven and place the pan on a wire rack to cool completely before frosting. If you try to frost a warm cake, the frosting will melt and slide right off.

Part 3: Making the Creamy Brown Sugar Frosting

1. Melt Butter and Brown Sugar: In a small saucepan, melt the ½ cup of unsalted butter over medium heat. Add the 1 cup of packed light brown sugar and stir until it’s completely dissolved and the mixture is smooth.
2. Boil the Base: Bring the mixture to a boil, then add the ¼ cup of milk or heavy cream. Return to a boil and cook for exactly 2 minutes, stirring constantly. This step creates a rich, caramel-like base for the frosting.
3. Cool Slightly: Remove the saucepan from the heat and stir in the 1 teaspoon of vanilla extract. Let the mixture cool for about 10-15 minutes, until it’s warm but not hot.
4. Whisk in Powdered Sugar: Transfer the brown sugar mixture to a mixing bowl. Using an electric hand mixer or a stand mixer, gradually beat in the sifted powdered sugar. Start with 1 ½ cups and add more as needed until you reach your desired spreading consistency. The frosting should be thick, creamy, and spreadable.
5. Frost the Cake: Once the cake is completely cool, spread the brown sugar frosting evenly over the top. You can make decorative swirls with the back of a spoon for a rustic look. Allow the frosting to set for about 30 minutes before slicing and serving.