Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt until well combined.

In a large mixing bowl, beat the softened butter and granulated sugar together using an electric mixer on medium speed for about 2-3 minutes until light and fluffy.

Mix in the egg, vanilla extract, and buttermilk to the creamed mixture until creamy and unified.

Gradually add the dry ingredients into the wet mixture, stirring gently until just combined.

Divide the dough into two equal parts and color one half pink and the other half blue using gel food coloring.

Roll small amounts of both colored doughs together into marbled balls and place them on the prepared baking sheet, spaced 2 inches apart.

Bake for 10-12 minutes or until edges are lightly golden, keeping a close eye to avoid overbaking.

Allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack.