Preheat oven to 300°F. Line baking sheets with parchment paper.
Cream butter in a large bowl with an electric mixer on medium-high speed until very light in texture, about 5min.
Add sugar & continue beating until sugar is completely combined, about 2-3min.
Combine flour & cornstarch in a separate large bowl. Add to butter mixture & mix with an electric mixer on low speed until just combined.
Place ball of dough by heaping tbsp on prepared baking sheets 2" apart. Press down with a fork.
Bake in preheated oven 20-25min or until bottoms are lightly golden. Remove cookies from baking sheet & cool on wire racks.
