Whisk together the flour, baking powder and salt in a small bowl. Beat the sugar and butter in the bowl of an electric mixer fitted with a paddle attachment on medium speed until light and fluffy, about 5 minutes. Beat in the vanilla and egg. Turn the mixer to low, add the flour mixture and mix until completely incorporated.
Remove the dough from the mixer and divide into 4 equal pieces. Add 1 piece back into the mixer. Turn the mixer to low speed and color the dough with the purple gel food coloring, about 4 drops, until completely incorporated. Wipe the bowl and the paddle clean and repeat with the remaining 3 pieces, coloring one neon green, one black and one orange.
Starting with the purple dough, tear half of it into teaspoon-size pieces and scatter them on top of a piece of parchment. Repeat with the remaining colors, tearing half of each color into teaspoon-size pieces and filling in all the spaces so that you have an oval shape with barely any of the parchment showing underneath. Top with another piece of parchment and use a rolling pin to roll the colors together until it is one big piece of dough about 1/4 inch thick. Repeat with the remaining dough. Refrigerate both parchment-covered pieces of dough until chilled, about 1 hour.
Position oven racks in the top and bottom thirds of the oven and preheat to 350 degrees F. Line 2 baking sheets with parchment.
Remove the top piece of parchment from one piece of dough and cut out circles with a 2-inch circle cookie cutter. Place on one of the prepared baking sheets about 2 inches apart. Repeat with the other piece of dough and baking sheet. Bake until the edges are just starting to turn golden, 10 to 12 minutes. Let cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely, about 1 hour.
Spread a small amount of frosting onto the edge of the cookies and roll them in the orange and black sanding sugar until completely coated.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
Copyright 2018 Television Food Network, G.P. All rights reserved.