Preheat oven to 350F
Line a baking sheet with parchment paper.
In mixer, cream butter, oil, sugars to combine.
Add egg and vanilla, mixing.
Add in the flour, cornstarch, baking powder, and salt until a dough has formed.
Divide into 10 equal pieces and roll into balls. (2.5 ounces each)
Place on parchment and press down with the bottom of a measuring cup.
Bake 10-13 mins until bottoms get slightly golden.