Creating a marble cake is a fun and rewarding process. Follow these steps to bake a perfect marble cake:

1. Preheat the Oven: Begin by preheating your oven to 350°F (175°C). Grease a 9-inch (23 cm) loaf pan or a similar-sized cake pan, and set it aside.
2. Mix Dry Ingredients: In a medium-sized bowl, sift together the flour, baking powder, and salt. This ensures that the ingredients are evenly distributed and helps create a light, airy cake.
3. Cream Butter and Sugar: In a large mixing bowl, use an electric mixer to cream the softened butter and granulated sugar together until light and fluffy. This process should take about 3-5 minutes.
4. Add Eggs and Vanilla: Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract to infuse the batter with its sweet aroma.
5. Incorporate Dry Ingredients and Milk: Gradually add the dry ingredients to the butter mixture, alternating with the milk. Begin and end with the dry ingredients, mixing until just combined. Be careful not to overmix, as this can result in a dense cake.
6. Separate the Batter: Divide the batter into two equal parts. To one half, add the cocoa powder and mix until fully incorporated. This creates the chocolate portion of the marble cake.
7. Create the Marble Effect: Pour half of the vanilla batter into the prepared cake pan. Then, add half of the chocolate batter on top. Repeat the process with the remaining batters. Use a knife or skewer to gently swirl the batters together, creating a marbled effect.
8. Bake: Place the cake in the preheated oven and bake for 45-55 minutes, or until a toothpick inserted into the center comes out clean.
9. Cool and Serve: Allow the cake to cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely before slicing and serving.