Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.
In a medium bowl, whisk together the flour, baking powder, and salt. Set it aside.
In a large bowl (or your stand mixer), beat the softened butter and granulated sugar together until it’s light and fluffy.
Add the eggs one at a time, beating well after each addition. Then, stir in the vanilla extract.
Gradually add the dry ingredients in three additions, alternating with the milk in two additions. Start and end with the dry ingredients. Mix until just combined.
Divide your batter in half into two separate bowls. In one bowl, whisk the cocoa powder with the hot water until smooth, then gently fold this chocolate mixture into one half of the batter.
Alternate spoonfuls of plain vanilla batter and chocolate batter into your prepared cupcake liners. Don’t fill them more than two-thirds full. Use a butter knife or a skewer to gently swirl the two batters together.
Bake for 18-22 minutes, or until a wooden skewer inserted into the center comes out clean.
Let them cool in the muffin tin for a few minutes, then transfer them to a wire rack to cool completely.
