Rocky Road Brownie Bars

2 cups of Semisweet Chocolate Chips
1 1/4 cups of All Purpose Flour
2 Tbsp of Cocoa Powder
1 ½ tsp of Baking Powder
¾ cup of Granulated Sugar
1/4 cup of Brown Sugar
6 Tbsp of Unsalted Butter, at room temperature
Pinch of Salt
1 tsp of Vanilla Extract
2 Eggs
2 cups of Mini Marshmallows
1 ½ cups of Walnuts, coarsely chopped