In a large bowl, stir and mash butter and bananas until well combined. Stir in Stevia, granulated sugar then egg, vanilla and peanut butter until well combined. Slowly stir in flour, baking soda, salt and chocolate chips. Stir until dough forms then transfer to prepared baking dish. Bake for 30-35 minutes, or until golden and baked through. Let cool completely, drizzle with melted chocolate and cut into squares to serve.