Whisk flour, baking powder, and salt. Beat butter and sugar until light. Mix in egg and extracts. Add dry ingredients in parts, then chill dough 1–2 hours.
Preheat oven to 175°C / 350°F. Roll out dough, cut into hearts, and place on baking sheets.
Bake 8–11 minutes until edges are golden. Cool completely.
Mix powdered sugar, milk, corn syrup, vanilla, and salt for icing. Tint with gel colors. Pipe outlines and flood centers. Decorate with sprinkles.
Let cookies dry several hours or overnight before stacking.