Microwave for 30 seconds and then remove and stir. Repeat, stirring every 30 seconds until the chocolate chips and butter start to melt.
Stir until the chocolate chips are completely melted and smooth. Add the powdered sugar and stir again until smooth.
1/2 cup powdered sugar
Pour the ganache over top of the cooled brownies and spread out to the edges. Let cool and cut into squares.
Notes
This makes thin, fudgy brownies. You could make the brownies in a smaller 9 x 9 pan. That would make thicker brownies and need a longer cooking time of 35-40 minutes.