2 sticks (1 cup) unsalted butter, cut into 16 pieces
1 large egg, lightly beaten
3.75 to 4 cups all-purpose flour
1/2 tsp. kosher salt
Decorating icing and assembly:
1 (1 lb.) box powdered sugar
4 tsp. powdered egg whites (not reconstituted)
1 Tbsp. fresh lemon juice
1 tsp. vanilla extract
Gel food coloring (optional)
Sprinkles, dragées, and sanding sugar, for decorating
Special Equipment: Rolling pin; assorted 2- to 3-inch cookie cutters (in shapes such as people, snowflakes, candy canes, etc.); a metal offset spatula; a pastry bag fitted with ⅛- to ¼-inch plain tip (optional)