Combine flour, oats, white sugar, brown sugar, butter, cornstarch, cinnamon, baking powder, and salt with your hands or a fork until a soft crumble forms. Set aside.
1/2 cup unsalted butter, 3/4 cups rolled oats, 1 cup all-purpose flour, 1/2 tsp cinnamon, 1/4 tsp salt, 1/4 tsp baking powder, 1/4 cup dark brown sugar, 1/4 cup white sugar