Whip butter with an electric mixer until fluffy, about 5 min. Stir in lavender extract, powdered sugar, cornstarch and flour, and beat on low for 1 min., then on high for 3 to 4 min. Stir in lavender flowers. Drop cookies by heaping teaspoons about 2 inches apart on an ungreased cookie sheet. Bake for 10 to 13 min., watching that the edges don't brown too much. Cool cookies on wire racks.