Preheat oven to 350°F (175°C). Line a baking pan with parchment paper.
Mix softened butter and sugar until creamy. Add egg and vanilla, mix well.
Gradually add flour, baking powder, and salt. Press dough into the prepared pan.
Bake for 20-25 minutes until golden. Let cool.
Prepare frosting by beating butter, vanilla, heavy cream, and powdered sugar until smooth. Spread over cooled bars and decorate with candy corn and sprinkles.
Notes
For best results, chill the frosting before spreading.