I found this recipe a couple of years ago, though I can't remember where it is from. These taste just like the thin mint cookies that the Girl Scouts sell.
Peppermint Fudge Cookie
3/4 c. packed brown sugar
3/4 c. granulated sugar
1/2 c. butter, softened
1/2 c. shortening
1 tsp. vanilla
2 oz. unsweetened baking chocolate, melted and cooled
2 1/4 c. all-purpose flour
1 tsp. baking soda
1 pkg. Andes creme de menthe chocolate mint chips
1 1/2 c. semisweet chocolate chips
Heat oven to 350 degrees.
In a large bowl, beat brown sugar, granulated sugar, butter, shortening, vanilla, and eggs with electric mixer on medium high until smooth.
Beat in melted chocolate.
On low speed, beat in flour and baking soda until well-blended.
Reserve 1 c. of the mint chips. Stir in remaining mints and the chocolate chips into dough.
On ungreased cookie sheets, drop dough by tablespoonfuls 2 inches apart.
Baking 10 minutes or until centers of cookies no longer look moist (do not overbake).
Immediately sprinkle with reserved mint chips.
Cool one minute; remove from cookie sheets to cooling racks.
Makes about 6 dozen.