Preheat the oven to 325 degrees F. Line a 9x9 inch baking pan with foil, extending the foil over the edges of the pan. Coat the foil with nonstick cooking spray; set the pan aside.
In a medium saucepan heat and stir the butter, milk chocolate and unsweetened chocolate over low heat until melted and smooth; cool slightly. Stir in the sugar and vanilla until combined. Add the eggs, one at a time, beating with a wooden spoon after each addition.
In a small bowl, stir together the flour, baking powder and salt. Add the flour mixture to the chocolate mixture; stir just until combined. Pour the batter into the prepared baking pan, spreading evenly.
In a small bowl, stir together the flour, baking powder and salt. Add the flour mixture to the chocolate mixture; stir just until combined. Pour the batter into the prepared baking pan, spreading evenly.
Bake for 25 minutes. Break the truffle squares into irregular-shaped pieces. Sprinkle over warm brownies. Cool in pan on a wire rack. Using the edges of the foil, lift uncut brownies out of pan. Cut into bars.
Notes
For the truffle squares, I used the Ghirardelli brand, found in my grocery store's candy aisle.