Trail mix ingredients such as salted peanuts, pretzel sticks, dried cranberries, raisins and chocolate pieces make a sweet-and-salty topper for brownies.
Line a 13x9x2-inch baking pan with heavy foil, extending foil over edges of the pan. Butter the foil; set pan aside.
In a heavy large saucepan, heat 1 cup butter and the chocolates over low heat, stirring constantly, until chocolate is melted and smooth. Set aside to cool slightly. In a medium bowl, stir together eggs, granulated sugar, brown sugar and vanilla. Stir into warm chocolate mixture; cool to room temperature.
In a small bowl, stir together flour, baking powder and salt. Fold flour mixture into chocolate mixture. Spread the batter in the prepared pan.
Sprinkle about 1 1/2 cups trail mix ingredients over brownie batter in pan.
Bake in a 350° oven for 35 to 40 minutes or until brownies appear shiny, begin to crack on top and appear set. (Do not overbake.) Cool completely in pan on a wire rack; then cut into bars. (For cleaner cuts, you can chill brownies in pan in refrigerator for several hours. Use foil to lift chilled brownies out of pan before cutting.)