Preheat oven to 350°F (175°C). Grease and line a loaf pan with parchment paper.
In a bowl, whisk flour, baking powder, and salt. Set aside.
Beat butter and sugar until light and fluffy (~3 mins). Add eggs (one at a time) and vanilla, mixing well.
Alternately add dry ingredients and milk, starting and ending with flour. Mix until just combined.
Take ⅓ of the batter in a separate bowl. Mix cocoa powder + hot water into it until smooth.
Spoon vanilla and chocolate batters alternately into the pan. Swirl gently with a knife.
Bake for 40-45 mins or until a toothpick comes out clean. Cool in pan for 10 mins, then transfer to a wire rack.
