Preheat your oven to 350°F. Line two baking sheets with parchment paper.

In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.

Add the egg and vanilla extract, mixing until well combined.

Gradually add the dry ingredients to the butter mixture, mixing until just combined.

Divide the dough in half. To one half, add the cocoa powder and milk, mixing until fully incorporated.

Using a tablespoon, scoop out a ball of the vanilla dough and a ball of the chocolate dough. Roll them together to form a larger ball, then place it on the prepared baking sheet.

Repeat with the remaining dough, spacing the cookies about 2 inches apart.

Bake for 10-12 minutes, or until the edges are lightly golden.

Let the cookies cool on the baking sheets for 5 minutes before transferring them to wire racks to cool completely.