1/2 cup unsalted butter (one stick), melted
1 cup granulated sugar
1/3 cup light brown sugar
6 ounces Greek yogurt (I used plain non-fat yogurt, but you can use vanilla or chocolate)
1 Tablespoon vanilla
1 cup mashed bananas (about 3 bananas)
2/3 cup unsweetened cocoa powder
3/4 cup flour
3/4 teaspoon baking soda
1/2 teaspoon salt
Stir together melted butter, granulated sugar and brown sugar. Add in egg, yogurt, vanilla and mashed bananas. Stir to combine.
Mix cocoa powder, flour, baking soda and salt in until well combined.
Grease a 9x9 inch baking dish with butter or non-stick spray. Pour in the batter, and smooth the top.
Bake at 350* for 40-45 minutes.
Let cool, then cut into squares.