Pour the butter, mashed bananas, eggs, vanilla extract, baking soda and salt into the bottom of your stand mixer bowl. Mix over medium setting for about 30 seconds, or until incorporated fully.
Add the gluten free rice flour blend and cocoa powder. Mix for an additional 45 seconds to 1 minute, or until incorporated fully.
Pour the batter into a greased 12-inch cast iron skillet.
Drop the melted peanut butter by tablespoons on top of the batter. Use the tip of a knife to swirl the peanut butter into the design of your choice.
Bake for 40 to 45 minutes, or until cooked through.
Let cool for at least 3 hours, but up to 24 hours, before serving.