Preheat oven to 350°F and line or grease a 9x9-inch baking pan.
Mix wet ingredients by whisking together melted butter, granulated sugar, and brown sugar for 1–2 minutes until smooth.
Add eggs and vanilla and mix until fully combined.
Fold in dry ingredients (flour, cornstarch, baking powder, salt) until just combined. Do not overmix.
Spread batter evenly into prepared pan.
Bake for 25–30 minutes, until the top is lightly golden and a toothpick comes out clean.
Cool completely for at least 1 hour.