
Bring the butter, eggs, and milk to room temperature. In a large bowl, beat the softened butter with 200 g of the sugar until pale, creamy, and slightly fluffy; set aside the remaining 50 g sugar for the chocolate batter.



Add the eggs one at a time, mixing well after each addition so the batter stays smooth. Pour in 150 ml of the milk and mix until combined; reserve the remaining 50 ml milk for later.



In a separate bowl, whisk together the flour and baking powder, then sift them into the wet mixture. Fold gently just until no dry streaks remain, taking care not to overmix.



Divide the batter evenly between two bowls. Stir the vanilla extract into one bowl to make the vanilla batter.



Add the cocoa powder, reserved 50 g sugar, and reserved 50 ml milk to the second bowl. Mix until the chocolate batter is evenly colored and smooth.



Grease and flour a loaf pan, or line it with parchment paper. Spoon in the vanilla and chocolate batters in layers or alternating dollops, then drag a skewer or knife through the batter a few times to create a marble pattern.



Preheat the oven to 390°F (200°C). Place the loaf in the oven, immediately reduce the temperature to 340°F (170°C), and bake for 50 to 60 minutes, or until a toothpick inserted in the center comes out clean. If the top browns too quickly, loosely cover it with foil.



Let the cake cool in the pan for about 10 minutes, then lift it out and transfer it to a wire rack. Cool completely before slicing for the cleanest texture.



Dust with powdered sugar just before serving, if desired. Slice and serve at room temperature.





