combine margarine, egg and Splenda on med speed until creamy. Add 2 tablespoons diet Mt. Dew (couldn''t find ingredient chart) and vanilla. Mix well. Slowly add flour and baking powder. Store in refrigerator at least 2 hours.
Roll out on floured surface and use cookie cutters. Bake in 400 degree oven for 6 to 10 minutes. Cool for a minute on pan then cool on cookie rack.