Preheat oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan.
In a medium bowl, whisk together flour, baking powder, and salt.
In a large bowl, cream butter and sugar until light and fluffy. Beat in eggs one at a time, then stir in vanilla.
Add flour mixture alternately with milk, mixing just until incorporated.
Divide batter into two bowls. Stir cocoa into one portion. Drop alternating spoonfuls of vanilla and chocolate batter into the prepared pan. Swirl with a knife for a marbled effect.
Bake for 45 minutes, or until a toothpick inserted into the center comes out clean. Cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.