Preheat your oven: Start by preheating your oven to 350°F (175°C). Grease a 9-inch bundt pan with butter or non-stick spray to ensure the cake releases easily after baking.
Prepare the dry ingredients: In a medium bowl, whisk together 2 cups of all-purpose flour, 1 1/2 teaspoons of baking powder, and 1/2 teaspoon of salt until evenly blended. This will help distribute the leavening agent and salt evenly through the flour.
Cream butter and sugar: In a large mixing bowl, use an electric mixer to cream together 1 cup of room temperature unsalted butter and 1 1/2 cups of sugar until the mixture is light and fluffy.
Add eggs and vanilla: Beat in 4 large eggs, one at a time, ensuring each egg is well incorporated before adding the next. Then mix in 2 teaspoons of vanilla extract.
Combine wet and dry ingredients: Gradually alternate adding the flour mixture and 3/4 cup of milk to the butter mixture. Start with a third of the flour, then add half the milk. Repeat this process, finishing with the flour mixture. This method helps to prevent overmixing and ensures a tender cake.
Prepare the chocolate batter: Divide the batter in half. In one half, mix in 1/4 cup of cocoa powder dissolved in 1/4 cup of boiling water. Stir until fully incorporated and the batter is smooth.
Marble the batters: Take turns spooning the vanilla and chocolate batters into the prepared bundt pan. Once all the batter is in, use a knife or a skewer to swirl the two batters together gently, creating a marbled effect.
Bake the cake: Place the bundt pan in the oven and bake for 50-60 minutes. To check for doneness, insert a toothpick into the center; it should come out clean.
Cool the cake: After baking, remove the cake from the oven and allow it to cool in the pan for about 10 minutes. Then, carefully transfer the cake to a wire rack to cool completely.