Whisk together flour, baking powder, and salt. Alternately beat in flour mixture with milk until blended.
Spread 1/3 of vanilla batter in bottom of loaf pan. Dollop with 1/2 of chocolate batter. Repeat with vanilla and remainder chocolate. Top with rest of vanilla batter.
Bake at 350°F for 1 hour 10 minutes. Cool completely, then slice and serve!