Preheat oven to 350F.

Line baking sheets with parchment paper.

Set aside a large dinner plate or cutting board (anything that won’t absorb color!!) on your work surface.

Whisk together the flour, baking powder, and salt in a medium bowl, then set aside.

In a bowl fitted to your stand-mixer, cream together the softened butter and sugar until light and fluffy. Add the egg yolk and vanilla, then mix until thoroughly combined.

Slowly add the flour mixture to the butter mixture, scraping down the sides of the bowl as needed, until all ingredients are thoroughly combined.

Turn off mixer (but leave bowl and paddle attached) and scrap out the cookie dough onto a clean (but un-floured) work surface. Shape and flatten dough into a thick disc, then use a knife to divide the disc into four equal parts.

Place Quarter 1 back in the mixing bowl and add the yellow food coloring, then mix together until color is thoroughly incorporated into the dough. Scrape the yellow dough out of bowl and form into a small ball, then place on the dinner plate/cutting board you set aside earlier.

Repeat the above process with Quarter 2 (red food coloring), Quarter 3 (green food coloring), and Quarter 4 (blue food coloring). IT IS IMPORTANT TO CREATE THE COLORED DOUGH IN THIS ORDER.

Use your hands to roll out each quarter of colored dough into a long rope on your work surface, making sure that each rope is around the same length as the others. Place the four colored ropes together, as if making a rainbow.

Take all four dough ropes and twist together into a single, twisted log of dough.

Wrap the log of cookie dough in plastic wrap and set in the fridge for 10-15 minutes.

Remove slightly chilled dough from the fridge, unwrap, and place the log on your (un-floured) clean work surface. Pinch off a 1-inch piece of dough from the log, then use your hands to roll it into a multicolored rope (about 8-9 inches long). Then, curl the rope up on itself like a snail shell or pinwheel and place on the lined baking sheet. Sprinkle with sanding sugar if desired. Repeat process with remaining dough.

Bake pinwheel cookies for 10-12 minutes at 350F. Let cool about 5 minutes on baking sheets, then remove to a wire rack to cool completely.