1. Preparing the Dough

1. Preheat the oven to 350°F (175°C).
2. In a large bowl, cream together the butter and sugar until light and fluffy.
3. Add the egg and vanilla extract, mixing until combined.
4. In a separate bowl, whisk the flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just incorporated.

2. Dividing and Coloring the Dough

1. Divide the dough into 4–6 equal portions.
2. Add a few drops of gel food coloring to each portion and knead the dough until the color is evenly distributed.
3. Flatten each portion into a disc, wrap them in plastic wrap, and chill for at least 30 minutes.

3. Creating the Marble Effect

1. Roll out small portions of each colored dough and stack or twist them together.
2. Gently roll the combined dough into balls and flatten them slightly to form marbled cookies.

4. Baking the Cookies

1. Arrange the marbled cookie dough on a baking sheet lined with parchment paper.
2. Bake for 8-10 minutes, or until the edges are lightly golden.
3. Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack.