Combine butter, granulated sugar, and powdered sugar in a mixer and beat well until light and fluffy.

Add in the eggs, vanilla, and any flavoring (if desired), scraping the bottom and sides of the bowl.

Mix in the flour, baking powder, and salt until combined. Divide the dough into three even sized balls.

Return ⅓ of the dough to the mixing bowl and add a 5-10 drops of red gel food coloring, mixing well until the dough is evenly colored. Repeat with another ⅓ of the dough and the green food coloring.

Flatten each ball of dough into a disc and wrap it in plastic wrap. Chill in the freezer for 20 minutes or in the fridge for 1 hour. 

Roll out each disc of dough, one at a time, on a piece of parchment paper to create rectangles that are roughly the same size and shape. Stack the thin layers of dough on top of each other with white in the middle and red and green on the top and bottom. Trim the long sides so that the edges are even. The shorter ends can be trimmed later after you roll the dough into a log.

Carefully roll the dough into a nice, tight log, using the bottom piece of parchment paper to lift the dough and help you roll it to avoid cracks. Then wrap the dough in plastic wrap and chill in the fridge for at least 1-2 hours. 

Once the dough is chilled, roll in nonpareils on a baking sheet to coat the outside of each cookie. You really have to press the dough into the sprinkles to get them to stick and give good coverage. At this point, you can slice and bake the cookies, or wrap them up and store them in the fridge for up to 1 week to slice and bake as needed.

When ready to bake your pinwheel cookies, preheat the oven to 350 degrees F. Line a baking sheet with parchment paper.

Unwrap the roll of cookie dough and cut off the uneven ends. Then slice into ¼-inch cookies using a sharp knife and space them 1-2 inches apart on the parchment paper.

Bake for 10-12 minutes until set, then cool completely on a wire rack.