For the cookie dough: Whisk together the flour, baking powder and salt in a medium bowl.
Add the butter and granulated sugar to the bowl of a stand mixer fitted with a paddle attachment. Beat on medium speed, scraping down the bowl at least once, until light and fluffy, about 2 minutes. Add the vanilla and 1 egg and beat to combine. Add the dry ingredients and beat on low speed until incorporated. Scrape down the bowl and continue to beat for 30 seconds.
Remove 1/3 cup of the dough to a small bowl. Add about 1/4 teaspoon black food coloring and stir to combine. Cover with plastic wrap and refrigerate until slightly firm, about 30 minutes.
Add about 1/2 teaspoon red food coloring to the dough remaining in the bowl of the stand mixer. Mix on medium speed until combined, about 1 minute, scraping down the bowl at least once. Cover with plastic wrap and refrigerate until slightly firm, about 30 minutes.
Place the red dough on a lightly floured clean work surface and roll it into a 7-inch log with a 2-inch diameter. Wrap with plastic wrap and place back in the refrigerator for 15 minutes.
Place the black dough on a lightly floured clean work surface and roll or flatten it into a 7-by-2-inch rectangle, 1/4-inch thick. Wrap with plastic wrap and place on a baking sheet and refrigerate for 15 minutes.
Whisk together the remaining egg with 1 tablespoon water in a small bowl for the egg wash.
Remove both doughs from the refrigerator. Slice the red log in half lengthwise. Lightly brush the flat end of one half with egg wash. Place the black dough onto it to adhere. Lightly brush the flat end of the other red half with egg wash and place it on top of the black dough. (If the dough softens and becomes difficult to work with, simply place it in the refrigerator for 10 minutes.) Wrap the log with plastic wrap and lightly roll it back and forth to round out the sides. Refrigerate the log for 1 hour.
Preheat the oven to 350 degrees F. Line 2 baking sheets with parchment paper.
Trim about 1/4 inch from each end of the log to reveal an even and flat Santa. Cut the log into 24 equal slices that are about 1/4-inch thick. Make sure to clean the knife after each slice for a clean cut so the colors won't bleed on each other. (If the log softens and becomes difficult to work with, simply place it in the refrigerator for 10 minutes.)
Arrange 12 cookies evenly on each prepared baking sheet. Bake until the edges are lightly golden, rotating the baking sheet halfway through, 14 to 17 minutes. Let cool completely on the baking sheets.
For the royal icing: Add the confectioners' sugar, meringue powder and 1 tablespoon water to the bowl of a stand mixer fitted with a whisk attachment. Beat on high for 1 minute, scraping down the sides as needed. If the icing is too thick, add 1 more teaspoon of water and beat until combined. Transfer to a small pastry bag fitted with a small round tip (size 4).
Pipe a white collar on the top half of the cookie. Sprinkle it with white sprinkles. Pipe buttons on the red coat and the black belt. Pipe a square buckle in the middle of the black belt.
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.
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