Step 1

For the cookies: Whisk together the flour, baking powder and salt in a small bowl.

Step 2

Beat the superfine sugar and butter with an electric mixer on medium speed in a large bowl until light and fluffy, about 5 minutes; beat in the egg, then the vanilla. Add the flour mixture and mix on medium-low speed until completely incorporated. Divide the dough in half, pat into 2 discs about 1/4 inch thick, wrap in plastic wrap and refrigerate until firm, about 1 hour.

Step 3

Position oven racks in the top and bottom thirds of the oven and preheat to 350 degrees F. Line 2 baking sheets with parchment paper.

Step 4

Let the dough sit at room temperature for a few minutes to make rolling easier. Roll out 1 disc of dough at a time between 2 pieces of parchment paper until 1/8 inch thick. Cut out shapes with cookie cutters and arrange about 2 inches apart on the prepared baking sheets.

Step 5

Bake until the cookies are golden brown on the bottom, 10 to 12 minutes. Let the cookies cool on the baking sheets until firm enough to transfer to a wire rack; let cool completely.

Step 6

Gently gather any scraps of dough into a ball and press into a disc; wrap in plastic wrap and refrigerate the disc until firm enough to roll, about 1 hour. Cut out as many cookies as possible and bake.

Step 7

For the royal icing: Beat the confectioners' sugar, meringue powder and 1/3 cup water with an electric mixer on medium-high speed in a large bowl until stiff peaks form. Beat in the food coloring if using. (The icing can be stored in an airtight container at room temperature for up to 2 days.)

Step 8

Decorate the cookies with the icing; top with decorating sugar and sprinkles if using.