For the Crumbly Base and Topping:

• 2 cups (250g) All-Purpose Flour: The foundation of our bars, all-purpose flour provides structure and a tender crumb. When measuring flour, it’s crucial to do it correctly. Spoon the flour into your measuring cup and level it off with a straight edge. Avoid scooping directly from the bag, as this can compact the flour and lead to using too much, resulting in a denser, drier base. For a slightly more tender crumb, you could substitute a quarter of the all-purpose flour with cake flour, but all-purpose works beautifully for this recipe. If you are looking for a gluten-free option, a good quality gluten-free all-purpose blend can be used, but you may need to adjust the liquid slightly as gluten-free flours can absorb moisture differently.
• 1 cup (2 sticks or 226g) Cold Unsalted Butter, cut into small cubes: Cold butter is absolutely essential for achieving that signature crumbly texture in both the base and topping. The cold butter, when mixed with the flour, creates small pockets of fat. As the bars bake, the water in the butter turns to steam, creating air pockets and resulting in a light, flaky, and crumbly texture rather than a tough, doughy one. Ensure your butter is truly cold right up until you use it. You can even cube it and place it back in the refrigerator while you prepare the other ingredients. Unsalted butter is preferred as it allows you to control the salt content of the recipe precisely. If you only have salted butter, reduce the added salt in the recipe by ¼ teaspoon.
• ½ cup (100g) Granulated Sugar: Granulated sugar adds sweetness to the base and topping, but it also contributes to the texture. It helps to tenderize the gluten in the flour, resulting in a softer crumb. The sugar also caramelizes slightly during baking, adding a subtle depth of flavor and golden-brown color. You can use caster sugar for an even finer texture, but regular granulated sugar works perfectly well.
• ½ teaspoon Salt: Salt is a flavor enhancer; it doesn’t just make things taste salty. In sweet recipes, salt balances the sweetness and enhances the other flavors, bringing out the nuances of the butter, sugar, and rhubarb. Don’t skip the salt; it’s a crucial component for a well-rounded flavor profile. Use fine sea salt or table salt.

For the Rhubarb Filling:

• 4 cups (about 1 pound or 450g) Fresh Rhubarb, trimmed and cut into ½-inch pieces: Rhubarb is the star of the show, providing that signature tart and tangy flavor that complements the sweetness and richness of the custard. Choose firm, crisp rhubarb stalks. The color of rhubarb can range from green to deep red, but the color doesn’t necessarily indicate ripeness or flavor intensity. However, redder stalks are often considered more visually appealing. Trim off the leafy ends (rhubarb leaves are toxic and should never be eaten) and the root end. Cut the stalks into uniform ½-inch pieces for even cooking. If you can’t find fresh rhubarb, frozen rhubarb can be used. Thaw it and drain off any excess liquid before using.
• ¾ cup (150g) Granulated Sugar: Sugar is needed to balance the tartness of the rhubarb. The amount of sugar can be adjusted slightly depending on your preference and the tartness of your rhubarb. If your rhubarb is particularly tart, you might want to increase the sugar by a tablespoon or two. Conversely, if you prefer a less sweet filling, you can reduce it slightly.
• 2 tablespoons Cornstarch: Cornstarch acts as a thickener for the rhubarb filling. As the bars bake, the rhubarb releases its juices. Cornstarch helps to absorb this liquid and create a slightly thickened, jam-like consistency rather than a watery filling. Make sure to toss the rhubarb with the cornstarch and sugar mixture thoroughly to ensure even distribution and prevent clumps. If you don’t have cornstarch, you can use tapioca starch or arrowroot powder as substitutes, using the same amount.
• 1 teaspoon Vanilla Extract: Vanilla extract enhances the flavor of the rhubarb and adds a touch of warmth to the filling. Use pure vanilla extract for the best flavor. Imitation vanilla extract can be used, but the flavor will be less complex.

For the Creamy Custard Topping:

• 3 Large Egg Yolks: Egg yolks are the key to a rich, creamy, and decadent custard. They provide richness, structure, and a beautiful golden color to the custard. Use large eggs for consistency in baking recipes. Separate the yolks carefully, ensuring no egg white gets into the yolks, as this can affect the custard’s texture.
• 1 can (14 ounces or 397g) Sweetened Condensed Milk: Sweetened condensed milk is a magic ingredient for custards. It provides sweetness, creaminess, and thickness in one go. It creates a smooth, velvety custard without the need for cooking a traditional custard base. Make sure to use sweetened condensed milk, not evaporated milk, as they are not interchangeable. Sweetened condensed milk is already sweetened and thickened, while evaporated milk is unsweetened and not as thick.
• ¼ cup (60ml) Heavy Cream or Half-and-Half: Heavy cream adds richness and luxuriousness to the custard. Half-and-half can be used for a slightly lighter custard while still maintaining a creamy texture. Using cream or half-and-half helps to thin out the sweetened condensed milk slightly and creates a smoother, pourable custard.
• 1 tablespoon Lemon Juice, Freshly Squeezed: Lemon juice brightens the flavor of the custard and cuts through the sweetness, adding a refreshing tang that complements the rhubarb beautifully. Freshly squeezed lemon juice is always preferred for its bright, clean flavor. Bottled lemon juice can be used in a pinch, but the flavor may not be as vibrant.
• ½ teaspoon Vanilla Extract: Just like in the rhubarb filling, vanilla extract enhances the overall flavor of the custard, adding a touch of warmth and complexity. Again, use pure vanilla extract for the best results.