Cream butter on high until light yellow. Turn speed to low and add in powdered sugar. When mixed, turn speed to high and add vanilla. Turn back to low speed and add flour, 1/3 cup a time until crumbly. Use hands to gather dough together. Knead dough on a lightly floured counter until nice and smooth. For shortbread pan: spray lightly with nonstick spray, firmly press dough into pan. For heavy baking sheet: turn baking sheet over, spray back of sheet with nonstick spray. With floured rolling pin, roll into a circle, 1/2” thick and 8” wide directly on baking sheet. Use hands to nudge into round shape with smooth edges. You may find it easier to wrap and refrigerate dough for 20 min before rolling.