Dessert in 1 Pan with Chocolate and Vanilla Marble (50 Minutes)

Instructions:

1. Preheat your oven to 350°F (175°C). Generously grease a 9x13 inch rectangular baking pan and line the bottom with parchment paper.
2. In a large mixing bowl, sift together the all-purpose flour, 300g granulated sugar, baking powder, and salt.
3. Add the softened unsalted butter to the dry ingredients. Mix on low speed using an electric mixer until the mixture reaches a texture resembling coarse sand, ensuring the flour is fully coated in fat. (This is the 'reverse creaming' method for tenderness).
4. In a separate container, whisk together the eggs, whole milk, sour cream, and vanilla. Gradually add this liquid mixture to the dry ingredients, mixing until just combined and smooth.
5. In a small heat-proof bowl, bloom the cocoa powder by whisking it with 60ml boiling water and 25g granulated sugar until a smooth, dark paste forms. This will create a richer chocolate flavor and a slightly denser chocolate batter.
6. Transfer approximately 1.5 cups of the vanilla batter into the bowl with the chocolate paste and fold together until uniform.
7. Alternative dollops of the vanilla and chocolate batters into the prepared 9x13 pan. Use an offset spatula or butter knife to gently swirl the batters together to create a marble effect. Aim for gentle swirls to avoid overmixing and ensure distinct layers.
8. Bake for 35 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool in the pan before slicing into 15 squares.