Stir together flour, baking soda and salt. Add cocoa to dry mixture.
In mixing bowl, cream the butter with both sugars. Beat in the egg and vanilla. By hand or using low speed of mixer, stir in the flour mixture. Fold in the mint and chocolate.
Drop mixture by rounded tablespoonfuls onto ungreased cookie sheets. Bake 10-12 minutes. Cool 2 minutes on cookie sheet then transfer to rack to finish cooling.