Heat oven to 350. Grease 15x10x1-inch jelly roll pan.
In large bowl, combine all base ingredients at low speed until crumbly. Press in bottom of prepared pan. Bake at 350°F for 10 minutes. Cool completely.
Dissolve gelatin in boiling water; cool slightly. In large bowl, combine dissolved gelatin and 2 cups powdered sugar. Add 1/2 cup butter, shortening, peppermint extract and food coloring. beat 1 minute at medium speed or until smooth and creamy. Blend in remaining 2 cups powdered sugar until smooth. Spread filling evenly over cooled base.
In small saucepan over low heat, melt chocolate chips and 3 tb butter, stirring constantly until well blended. Spoon frosting over filling, carefully spreading to cover. Refrigerate until firm, cut into bars. Store in refrigerator.
Tip. for easier cutting, remove from refrigerator 20 minutes before serving.