Preheat oven to 350°F (175°C). Line baking sheets with parchment. Cream butter and maple sugar until fluffy, about 2 minutes.

Beat in egg and vanilla. Whisk flour, baking powder, salt, and cornstarch separately, then mix into the butter mixture until just combined.

Scoop 1.5-tablespoon balls of dough, roll them smooth, and place 2 inches apart on the sheets.

Bake 8 to 10 minutes until edges are golden and centers look slightly underdone. Let cool on the sheet for 5 minutes.

Transfer to a wire rack to cool completely; serve barely warm or at room temperature.