Preheat oven to 350 degrees. Prepare a loaf pan by buttering or use cooking spray.
Place butter and peanut butter into one bowl and use microwave to melt. Using a large mixing bowl and spooncombine peanut butter mixture and mashed bananas. Add both sugars, beaten egg and vanilla, stir to incorporate all ingredients. Then add flour, baking soda and salt. Stir just enough to combine, but do not over mix the batter.
Pour into prepared loaf pan. Bake at 350 degrees for 1 hour then test with toothpick. Loaf is done with toothpick comes out clean. Coolremove from pan, slice and enjoy!