4ouncesunsweetened chocolatechopped, melted, and cooled to room temperature
Adjust rack to center position in oven. Preheat oven to 375°F. Lightly spray a 9x13-inch pan with cooking spray.
In a medium bowl, whisk together flour, baking powder, and salt. Set aside. In a large bowl, use an electric mixer to whip butter until smooth. Beat in sugars at medium speed. Scrape bowl, increase to medium-high speed, and beat for 3 minutes or until light and fluffy. Blend in egg and vanilla until incorporated. Turn mixer to low and slowly stir in flour mixture, mixing until just combined and taking care not to overmix.
Turn dough into prepared pan and press into an even layer (you may lightly flour hands if it's sticking to fingers). Bake for approximately 13 to 15 minutes, or until just set and edges are beginning to show a hint of golden color. Set pan on a wire rack and allow to cool completely.
Once cookies are cool, prepare buttercream by using an electric mixer to beat butter until smooth, and then blend in powdered sugar alternated with milk. You may use more or less milk to achieve your desired consistency. Mix in vanilla and salt; beat at medium speed for 2 minutes. Reduce speed to low, pour in melted chocolate, and beat at medium speed for 1 more minute or until light and fluffy.
Evenly spread frosting over cooled sugar cookie bars. Slice into squares with a sharp knife, wiping clean between each cut. Store in an airtight container in the refrigerator. These are best if you allow them to come to room temperature before serving.
I have the most success with this recipe when I weigh my flour, rather than just using measuring cups. If you use measuring cups and your dough seems too sticky to press into the pan, add a tad bit more flour.
I melt my chocolate by placing it in a heat-proof bowl, microwaving on high for 30 seconds, stirring, and then repeating a couple times until solid all lumps melt when stirred. You may also melt chocolate over a double boiler.