Whisk flour, baking powder, and salt together in a bowl.
In another bowl, cream butter and sugar until light and fluffy, then beat in eggs one at a time.
Add vanilla, then mix in the dry ingredients alternately with milk until just combined.
In a small bowl, mix cocoa powder with hot water to form a smooth paste.
Stir in about ½ cup of the batter to make the chocolate mixture.
Line a muffin tin with cupcake liners.
Add spoonfuls of vanilla and chocolate batter alternately into each liner, then swirl lightly with a toothpick for the marble effect.
Bake at 350°F (175°C) for 18–22 minutes, or until a toothpick inserted in the center comes out clean.
Let cool on a wire rack before serving.
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