12 3/4 ounces very ripe mashed banana (1 1/2 cups)*
4 1/2 ounces creamy peanut butter (1/2 cup)
5 1/4 ounces nutella (1/2 cup)
Brown the butter: Warm a small, light-colored skillet on the stove over medium-low heat. Add the cut up butter and let it melt. Whisk frequently as it melts and starts to foam up. Once bubbling or foaming, whisk fairly constantly as the butter turns a rich hazel color. Once it gets to this point, remove from the heat and immediatley place on a cool surface. Whisk briefly for another 30 seconds or so, and then set aside to cool for 15 minutes.
Meanwhile, preheat oven to 375° F. Grease a 9″ x 5″ loaf pan with non-stick cooking spray.
In a medium to large bowl whisk together the all-purpose and whole wheat flours, baking powder, baking soda, and salt.
In a separate, large bowl, whisk together the cooled brown butter with the brown sugar until well-combined and lighter in color. Beat in the eggs, one at a time, until smooth. Beat in the vanilla extract and yogurt until smooth. Whisk in the mashed banana. Fold the dry ingredients into the wet until just combined, with the dry no longer visible; DO NOT OVERMIX!
Place the peanut butter and nutella in two separate, microwave-safe bowls and microwave for 5-10 seconds each, until they’ve reached a somewhat pourable consistency but aren’t too thin.
Pour half of the batter into the prepared loaf pan, making sure that it covers the entire bottom, and smoothing it in an even layer. Drizzle half each of the warm peanut butter and nutella evenly over the first layer of batter. Add the remaining batter, spreading it out to cover the first two layers and smoothing out the top. Then, drizzle over the remaining peanut butter and nutella evenly over the entire top surface of the bread batter. Use a firm wooden skewer or a knife to swirl the batter around briefly.
Bake the loaf in the preheated oven for about 1 hour to 1 hour 10 minutes, until a toothpick inserted into the center of the loaf (or a crack in the top) comes out clean. Cool in the loaf pan for about 15 minutes, and then carefully transfer to a wire rack to finish cooling – before slicing! Wrap tightly in plastic wrap and (preferably) store in an airtight ziploc-type bag. Slices of bread can be warmed up in the microwave for about 25 seconds. Serve warm or at room temperature.
*The banana doesn’t need to be pureed; just mash it by hand so there aren’t any huge chunks remaining and you can measure it out.