Beat butter in a large mixing bowl with an electric mixer on medium to high speed for 30 seconds. Add sugar and baking powder and beat until combined. Beat in egg, mint extract, and enough food coloring to tint light to medium green. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour with a wooden spoon. Divide dough in half. Cover and chill about 1 hour or until easy to handle.

Roll one half of dough between two sheets of waxed paper into an 8x7-inch rectangle. Peel off top sheet of waxed paper. Sprinkle half of crushed cookies evenly over dough to within 1/4 inch of all sides. Roll up, jelly-roll style, starting from one short side, removing bottom sheet of waxed paper as you roll. Pinch to seal. Repeat with remaining dough and crushed cookies. Wrap rolls in waxed paper or clear plastic wrap. Chill about 4 hours or until firm.

Remove one roll of dough from the refrigerator. Unwrap and reshape slightly, if necessary. Cut dough into 1/4-inch-thick slices. Place 2 inches apart on an ungreased cookie sheet.

Bake in a 375 degree F oven for 8 to 9 minutes or until edges are firm. Cool on cookie sheet for 1 minute. Remove and cool completely on wire racks. Repeat with remaining roll of dough. If desired, melt together chocolate and shortening in a heavy saucepan over low heat; drizzle over cookies. Makes about 50.

Wrap rolls of dough in plastic wrap. Place in a freezer container; freeze up to 1 month. To use, thaw just until soft enough to slice. Or, bake and cool cookies completely; do not drizzle with chocolate mixture. In an airtight or freezer container, arrange cookies in a single layer; cover with a sheet of waxed paper. Repeat layers, leaving enough air space to close container easily. Freeze up to 1 month. To use, thaw and drizzle with melted chocolate mixture.