Line 12.25” x 8.75” x 1” jelly roll pan with foil or parchment paper and spray with nonstick cooking spray. Set aside.
In a large bowl, cream butter and sugars together until light and fluffy. Stir in egg, vanilla and molasses. Whip 1-2 minutes or until it turns a light brown color. Stir in remaining ingredients and mix until dry ingredients are just combined. Press into prepared pan and bake 20-22 minutes or until edges are barely starting to brown. Cool completely.
For the frosting- whip all ingredients together until smooth. Spread overtop bars and cut into squares. Serve immediately or store in air tight container.
*if you don't have a small jellyroll pan, you can use a glass 8x8 dish, just add 2-3 extra minutes to the baking time.*