Beat the butter and sugar with an electric mixer until light and fluffy, about 2 minutes.

Add the egg and vanilla and beat until combined.

Add the flour and salt and mix on low just until no dry streaks remain.

Divide dough in half, shape into disks, wrap, and chill at least 1 hour.

Roll one disk to 1/4 inch thick on a floured surface. Cut shapes, place on parchment-lined baking sheets.

Bake at 350°F (175°C) for 8 to 10 minutes, until edges are barely golden. Cool on the sheet 5 minutes, then transfer to a rack.

Cool cookies completely, then decorate with icing and sprinkles. Let icing set before stacking.