Preheat oven to 350ºF. Prepare an 8-inch or 9-inch pan with nonstick spray or butter.
In a large saucepan over medium heat, melt butter. Cool slightly, then whisk in eggs, egg yolk and vanilla. Next, stir in brownie mix and baking powder. Stir mini chocolate into the brownie batter and spread into the prepared baking pan. In a small bowl, combine the butter, brown sugar and cinnamon; mix together with your fingers or a fork until mixture is mostly pea-sized crumbs. Sprinkle cinnamon sugar mixture over the top of the brownie batter.
Bake in a 350˚F oven for 25-30 minutes (or a little longer for n 8-inch pan). Allow brownies to cool completely before slicing.