Preheat oven to 350°F and grease a loaf tin with butter or nonstick spray. Set aside. Checkpoint: Your tin is ready when it glistens and has no dry spots.

In a large mixing bowl, whisk together 1 1/4 cups granulated sugar, 1 cup all-purpose flour, 1 1/2 teaspoons baking powder, and 4 tablespoons cocoa powder. Whisk for about 1 minute until all the cocoa powder is evenly distributed and no brown streaks remain. Checkpoint: Your dry mixture should be uniform in color with no cocoa clumps visible.

Pour 2/3 cup melted butter into the dry ingredients and add 1 teaspoon vanilla extract. Stir with a spatula for 1-2 minutes until all dry spots are moistened and the mixture looks like wet sand. Checkpoint: Every bit of flour should be coated with butter and the mixture should hold together when squeezed.

Crack 1 egg into the bowl and mix for 1 minute on medium speed or by hand until fully blended. Repeat with the second egg, mixing for 1 minute after adding it. The batter should be thick and fudgy at this point. Checkpoint: No streaks of egg should be visible in the batter.

Carefully pour 2/3 cup boiling water into the batter while whisking constantly for 2-3 minutes. The batter will transform into a thin, pourable mixture that looks like melted chocolate. This is normal! Checkpoint: Your batter should be smooth, shiny, and free of lumps.

Pour the batter into the prepared loaf tin and smooth the top with a spatula. Place the tin on the middle rack of the oven and bake at 350°F for 50 minutes, or until a toothpick inserted into the center comes out clean with no wet crumbs.

Remove the cake from the oven and let it cool in the tin for 15 minutes. The cake will pull away slightly from the sides as it cools.

Carefully turn the loaf tin on its side and gently slide the cake out onto a wire cooling rack. Let it cool completely for at least 30-45 minutes before adding chocolate.

Once cooled, melt 1 ounce white chocolate and 1 ounce milk chocolate in separate small bowls (in microwave in 20-second bursts, stirring between each burst, or in a double boiler over simmering water) for 1-2 minutes each until smooth and pourable.

Drizzle the melted white and milk chocolate over the top of the cooled cake in decorative lines or patterns using a spoon or squeeze bottle. Let the chocolate set for 10 minutes before slicing.