Preheat and Prepare Pan
Preheat your oven to 350°F (175°C). Grease a loaf or round cake pan and line with parchment paper.

Mix Dry Ingredients
In a bowl, whisk together the flour, baking powder, and salt. Set aside.

Cream Butter and Sugar
In another large bowl, beat the softened butter and sugar until light and fluffy, about 3–5 minutes.

Add Eggs and Vanilla
Add eggs one at a time, beating after each addition. Stir in the vanilla extract.

Combine Wet and Dry
Alternate adding the flour mixture and milk to the butter mixture, beginning and ending with flour. Mix just until combined.

Prepare Chocolate Batter
In a small bowl, stir cocoa powder and hot water until smooth. Fold in one-third of the vanilla batter.

Marble the Batter
Add spoonfuls of vanilla and chocolate batters alternately into the prepared pan. Use a skewer or knife to swirl through the batter.

Bake
Bake for 35–45 minutes, or until a toothpick inserted into the center comes out clean.

Cool and Serve
Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.