Preheat oven to 350 degrees F (175 degrees C). Line a baking sheet with parchment.

Beat sugar and butter together in a large bowl with an electric mixer until light and fluffy; beat in eggs, egg yolk, and vanilla extract. Whisk flour and baking powder together in a bowl. Stir flour mixture into butter mixture to form a fairly sticky dough.

Divide dough in half; with wet hands, shape dough into two loaves, about 1-inch high at the center, on the prepared baking sheet.

Bake in the preheated oven until lightly browned, about 20 minutes; remove to cool completely on a wire rack, about 30 minutes.

Slice loaves into 1/2-inch slices with a serrated knife. Place cookie slices back onto lined baking sheet, cut sides up.

Increase oven temperature to 450 degrees F (230 degrees C).