1. Into a large bowl, sift together flour, baking powder, and salt; set aside. Combine butter and granulated sugar in a mixing bowl; beat on medium speed until fluffy, about 3 minutes. Add egg and vanilla; mix well. With mixer on low speed, gradually add flour mixture; beat until combined.

2. Preheat oven to 350 degrees. Divide dough into 6 balls. Press balls on a floured work surface until about 4 or 5 inches in diameter, leaving edges raw to resemble rough cement. Transfer to a baking sheet. With lightly floured hands, let birthday child gently and evenly press each hand into a piece of dough to make hand print indentations. Sprinkle indentations with sanding sugar. Chill until firm, about 30 minutes.

3. Bake until edges of cookies are just turning golden, 15 to 18 minutes. Transfer to a wire rack to cool completely.