Step 1: Prepare the Oven and Pan

• Preheat your oven to 350°F (175°C). Grease and flour a 9×5-inch loaf pan or a 10-inch bundt pan, ensuring all sides are coated to prevent sticking.

Step 2: Mix the Dry Ingredients

• In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set this mixture aside for later.

Step 3: Cream the Butter and Sugar

• In a large mixing bowl, cream together the softened butter and granulated sugar using a hand mixer or stand mixer on medium speed. Beat for about 3-4 minutes until the mixture is light, fluffy, and pale in color.

Step 4: Add the Eggs and Vanilla

• Add the eggs to the butter mixture one at a time, beating well after each addition to ensure the eggs are fully incorporated. Stir in the vanilla extract and mix until combined.

Step 5: Alternate Adding Dry Ingredients and Milk

• Gradually add the flour mixture to the butter mixture in three parts, alternating with the milk. Start by adding a third of the flour mixture, mix until just combined, then add half of the milk. Repeat, finishing with the last third of the flour. Be careful not to overmix the batter—mix just until the ingredients are incorporated.

Step 6: Make the Chocolate Batter

• In a small bowl, mix the cocoa powder with 3 tablespoons of hot water to create a smooth chocolate paste.
• Transfer about 1 ½ cups of the vanilla batter to the bowl with the chocolate mixture and stir until fully combined, creating the chocolate batter.

Step 7: Create the Marble Effect

• Spoon the vanilla and chocolate batters alternately into the prepared pan, layering them as you go.
• 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 over-swirl, as you want distinct marble patterns throughout the cake.

Step 8: Bake the Cake

• Place the cake in the preheated oven and bake for 50-60 minutes for a loaf pan, or 45-55 minutes for a bundt pan. The cake is done when a toothpick inserted into the center comes out clean, or with just a few moist crumbs.
• If the top of the cake is browning too quickly, you can loosely cover it with aluminum foil during the last 10-15 minutes of baking.

Step 9: Cool and Finish

• Remove the cake from the oven and allow it to cool in the pan for 10-15 minutes. Then, carefully turn the cake out onto a wire rack to cool completely.
• Once cooled, dust the top of the cake with powdered sugar for a simple, elegant finish.