Sift together the flour, baking powder, and salt. In the bowl of a stand mixer, beat together the sugar and butter until fluffy. Add the egg, vanilla extract, and milk and beat until combined. Gradually add in the flour with the mixer on low speed. Beat until the dough pulls away from the side of the bowl.
Divide the dough in half, wrap in waxed paper and refrigerate for 2 hours.
Preheat oven to 375 degrees Fahrenheit
Sprinkle your work surface and your rolling pin with powdered sugar. Roll out one of the refrigerated dough packets to ¼-inch thickness. Cut into circles, then cut the circle in half.
Place on a greased cookie sheet, or parchment, or silicone baking mat. Bake for 7 to 9 minutes or until the cookies are just beginning to brown around the edges. Rotate the baking sheet midway through the baking time for more even baking.
Let sit on baking sheet for 2 minutes, then remove to a cooling rack.
Repeat with 2nd packet of cookie dough.
For the Royal Icing
Combine the ingredients in the bowl of a stand mixer fitted with the whisk attachment. Beat for 8 minutes on low speed. This batch is thicker for outlining the shapes (consistency should be like toothpaste).
For the watermelon colored cookies, we separated the icing into 3 bowls and colored one red and one green. We left the rest white for the thin rind of the watermelon. At this point, you can keep all of the icing this thickness, or you can separate 1/3 of the icing to use for outlining, then add a teaspoon of water at a time to achieve the best consistency for “flooding" the outlines with the thinner icing.
Transfer the icing to pastry bags. Outline the cookies in the appropriate colors, then fill the inside of the colors using a pastry tip as a way to spread the icing as you go.
Dot the red section with chocolate sprinkles to represent seeds before the icing sets.
