Preheat the Oven: Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan or line it with parchment paper to ensure the cake doesn’t stick during baking. A well-prepared pan helps the cake release smoothly after baking and ensures an even texture across the entire surface.
Prepare the Dry Ingredients: In a medium bowl, sift together the all-purpose flour, baking powder, and salt. Sifting is essential to remove any lumps from the flour and ensures even distribution of the baking powder, which is key to achieving a light and fluffy texture in the cake.
Cream the Butter and Sugar: In a large mixing bowl, use an electric mixer to cream the softened unsalted butter and granulated sugar together until the mixture is light and fluffy. This process should take about 3-4 minutes at medium speed. The butter and sugar should be fully incorporated, and the mixture should appear pale in color. This step is important because it helps create air pockets in the batter, which will result in a soft, moist cake.
Add the Eggs and Vanilla: Beat the eggs into the butter and sugar mixture one at a time, making sure each egg is fully incorporated before adding the next. Add the vanilla extract and continue mixing until everything is smooth and well combined. The eggs provide structure to the cake, while the vanilla enhances the flavor with a fragrant, sweet aroma.
Combine Wet and Dry Ingredients: Gradually add the sifted dry ingredients to the wet mixture in batches, alternating with the whole milk. Start with a small portion of the dry ingredients, then add a little milk, and repeat until all the dry ingredients and milk are incorporated. Mix gently to avoid overworking the batter, which could result in a denser texture. The batter should be smooth and thick, but pourable.
Prepare the Chocolate Swirl: In a small bowl, combine the cocoa powder with the hot water. Stir until the cocoa powder is completely dissolved, forming a smooth chocolate paste. This paste will be swirled into the vanilla cake batter to create the marble effect.
Marble the Cake Batter: Divide the cake batter evenly between two bowls. Add the cocoa paste to one of the bowls and mix it into the batter until fully incorporated. You should have one bowl of vanilla batter and one bowl of chocolate batter. To create the marble effect, spoon the vanilla and chocolate batters alternately into the prepared cake pan. Once all the batter is in the pan, use a butter knife or skewer to gently swirl the two batters together. Be careful not to overmix; you want distinct marble streaks in the finished cake.
Bake the Cake: Place the cake pan in the preheated oven and bake for 30-35 minutes. Start checking for doneness around the 30-minute mark by inserting a toothpick into the center of the cake. If it comes out clean or with only a few moist crumbs, the cake is done. If the toothpick comes out with wet batter, bake the cake for an additional 5-7 minutes and check again. The top should be golden brown and firm to the touch when fully baked.
Cool the Cake: Once the cake is baked, remove it from the oven and let it cool in the pan for about 10 minutes. This allows the cake to firm up slightly and makes it easier to remove from the pan without breaking. After 10 minutes, invert the cake onto a wire rack to cool completely before drizzling with chocolate sauce. Cooling the cake completely before adding the drizzle prevents the chocolate from melting too much and gives it a nice, shiny finish.
