Lightly coat a 9-inch square pan with nonstick cooking spray.
In a medium bowl, whisk together flour, baking powder, and salt.
In a separate bowl, stir together coconut oil, mashed banana, and vanilla.
Mix in Greek yogurt, stirring until no large lumps remain.
Mix in stevia and milk.
Add in flour mixture, stirring just until incorporated.
Fold in 2 1⁄2 Tbsp chocolate chips.
Spread batter into prepared pan and sprinkle remaining chocolate chips on top.
Bake at 300 F for 17-20 minutes. Cool completely to room temperature in pan, and let brownies rest for an additional 3 hours before slicing for the fudgiest texture.