Preheat oven to 350 degrees. Line muffin tins with paper cups and spray with non-stick spray. This recipe makes about 16-18 regular size muffins. (You could also use 2 loaf pans, if you prefer)
Mash 3 ripe bananas and set aside.
Melt coconut oil. In the bowl of an electric mixer, combine coconut oil and sugar. Beat on medium speed until it is well mixed. Add eggs one at a time, beating on medium-low speed. Add vanilla.
In a separate bowl, combine rice flour, oat flour, arrowroot, baking powder, baking soda and salt. Mix together with a fork to break up any lumps. Add wet mixture to dry, alternating with milk. Take care not to over stir.
Fill muffin cups about ½ full. In a microwave safe container, melt the peanut butter so it is easy to pour. Place a little dab of peanut butter in each muffin tin. Swirl around with a toothpick or butter knife. Top with remaining batter and then remaining peanut butter. Repeat swirling. Add additional toppings if you'd like - I used oats, banana slices and mini dark chocolate chips.
Bake for 25-30 minutes until golden brown and toothpick inserted comes out clean.