Preheat oven to 350 degrees. In large bowl, whip butter and sugar until light and fluffy. Stir in egg and vanilla and beat until pale yellow, 2 minutes. Gently stir in all dry ingredients until just incorporated. Scrape sides and mix again. Fold in chopped Mint Truffle Hershey Kisses and drop by the tablespoon full onto a greased cookie sheet. (about 8 cookies per sheet) Bake 8-9 minutes. Remove from oven, cool 3 minutes then transfer cookies to cooling rack.