Begin by preheating your oven to 350°F and greasing a loaf tin to prepare for baking. In a large baking bowl, whisk together the dry ingredients: 1 cup all-purpose flour, 1 1/2 teaspoons baking powder, and 4 tablespoons cocoa powder. Pour in 2/3 cup melted butter and 1 teaspoon vanilla extract, mixing well to combine. Add 2 eggs one at a time, mixing thoroughly after each addition to ascertain proper incorporation and a smooth, emulsified batter.
Once the eggs are fully incorporated, cautiously pour in 2/3 cup boiling water while whisking continuously until you achieve a smooth, thin batter. The boiling water might seem counterintuitive, but it's vital for creating the moist, tender crumb that distinguishes this pound cake from denser varieties. Pour the batter into your prepared loaf tin and bake for 50 minutes, or until a toothpick inserted into the center comes out clean, indicating the cake is fully cooked through.
Allow the cake to cool in the tin for 15 minutes before carefully removing it from the pan and transferring it to a wire rack to cool completely. Once cooled, drizzle the top with 1 ounce melted white chocolate and 1 ounce melted milk chocolate for a beautiful finish that adds layers of chocolate flavor without overwhelming the cocoa-rich base.