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

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

Cream the softened butter and sugar in a large bowl with an electric mixer until light and fluffy.

Add the egg and vanilla extract, beating until fully incorporated.

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

Divide the dough into portions, adding gel food coloring to each and knead until color is distributed.

Shape the dough into balls, flatten them slightly, stack them, and swirl to create a marbled effect.

Place them on prepared sheets, spacing about 2 inches apart, and bake for 10-12 minutes until edges are golden.

Cool on the baking sheet for 5 minutes before transferring to a wire rack.

Decorate with icing made from confectioners’ sugar and milk, then serve!