Preheat oven to 350 degrees. Grease an 8 inch square pan, then line with a piece of parchment paper cut to fit the bottom and extend over two sides. In a micro-safe bowl, melt margarine with sugars in the microwave. Cool slightly, then beat in egg and vanilla extract (and whiskey, if using). Stir together flour, baking powder, and salt, then mix into sugar mixture until just combined.
Fold in add-ins of your choice, one or a combination of several, and spread mixture into the prepared pan (added in bittersweet chips, dried cherries, and whiskey). Bake for 17-20 minutes, until a toothpick inserted in the centered tests nearly clean. Cool in pan, then use the parchment to remove from pan and slice into bars.
NOTE
This recipe doubles beautifully into a 9x13 inch pan.