1. Whisk Dry Ingredients: In the bowl of your stand mixer, combine the flour, baking powder, salt, powdered sugar, and granulated sugar. Mix with the paddle attachment on medium speed until everything is well blended.
2. Form Dough: Gradually add the softened butter chunks and vanilla extract. Continue mixing until the dough comes together and forms a soft ball.
3. Divide & Color: Remove all the dough and divide it into two equal portions. Place one portion back into the mixer, add the red food coloring, and mix until the dough is uniformly vibrant red.
4. Roll Out Dough: Take each dough portion and place it between two sheets of wax paper. Using a rolling pin, roll each out to an approximate 11x9 inch rectangle, about ¼ inch thick. Slide both rolled dough sheets (still between wax paper) onto a baking sheet and chill in the refrigerator for at least one hour until they are firm.
5. Layer Dough: Once firm, carefully peel off the top sheets of wax paper from both dough rectangles. Lightly brush the plain vanilla dough with a thin layer of water using a pastry brush. Gently flip the red dough onto the vanilla dough, aligning them perfectly. Lightly press the edges with your fingertips to seal them together, then trim any uneven edges with a small knife for a neat, straight rectangle.
6. Create Swirl: Make sure your layered dough is still cold but flexible. Starting from one of the long sides, tightly roll the stacked dough into a cylinder, jelly-roll style, to create a beautiful spiral.
7. Add Sprinkles & Chill Again: Pour your Valentine's Day sprinkles onto a large platter. Gently lift the dough log onto the sprinkles and roll it until the entire log is evenly coated. Wrap the sprinkle-covered log tightly in plastic wrap and chill it in the refrigerator for several hours, or you can freeze it for up to three months.
8. Slice & Bake: Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper. Slice the chilled dough log into ¼ inch thick cookies. Place them on the prepared baking sheet. Bake for 15-17 minutes, or until the edges begin to turn a light golden brown. Let the cookies rest on the baking sheet for about 5 minutes before carefully transferring them to a wire rack to cool completely.