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

In a medium bowl, combine the all-purpose flour, baking powder, baking soda, and salt.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the egg, vanilla extract, and buttermilk to the mixture and mix until smooth.

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

Divide the cookie dough and color one half pink and the other blue using gel food coloring.

Roll small amounts of the pink and blue dough together to form marbled cookie balls and place them on the baking sheet.

Bake for 10-12 minutes until edges are lightly golden, and allow to cool for a few minutes.

Transfer cookies to a wire rack to cool completely before serving.