Beat butter, 1 1/2 cup white sugar, and confectioners' sugar together in a bowl using an electric mixer until smooth and creamy; mix in cornstarch and 2 cups flour. Make a well in the center of the dough; add egg and mix well. Knead the remaining 1 cup flour into dough using your hands until dough pulls away from the sides of the bowl.
Roll dough into walnut-sized balls.
Pour 1/4 cup white sugar into a bowl; dip top of each ball in sugar. Arrange balls on a baking sheet. Slightly flatten each ball using the bottom of a drinking glass.
Bake in the preheated oven until edges of cookies are lightly browned, about 5 minutes. Cool in the pans for 5 minutes before removing to cool completely on a wire rack.
