Preheat oven to 350 degrees. Line an 8 x 8 metal baking pan with parchment paper and set aside.
In a large bowl, whisk together cooled melted butter, both sugars and vanilla until combined.
Add eggs, one at a time, whisking vigorously after each addition until well incorporated.
Next, whisk in mashed bananas then set aside.
In a separate bowl, whisk together cocoa powder, salt, flour and baking powder.
Using a spoon, stir dry ingredients into wet ingredients until *just* combined. Avoid overmixing.
Fold in 1/2 cup of dark chocolate chips.
Pour batter into baking pan then top with remaining chocolate chips.
Bake brownies for 30 to 35 minutes or until the edges are crisp and a toothpick inserted into the center comes out with a few moist crumbs clinging to it. Avoid overbaking or brownies will be dry.
Remove pan from oven and place on a baking rack to cool completely before slicing. (If you're a stickler for clean slices, place brownies in the freezer for 20 minutes before cutting.)
Store brownies in an airtight container at room temperature up to 3 days.