Step 1

Grease an 8 1/2- x 4 1/2-inch loaf pan with butter (Don’t butter if using stoneware pan.) Place a rectangle of parchment paper on the bottom of the pan. Preheat oven to 350 degrees F. (If using whey protein and a metal pan, preheat oven to 325-degrees F.)

Step 2

In a medium bowl, combine flours, protein powder, baking soda and salt. Stir with a whisk to mix. Set aside.

Step 3

In a large bowl, place sugar and butter, and beat with a mixer at medium speed until well blended and light in color, about 2 minutes. Add the eggs, one at a time, beating well after each addition. Add mashed banana, yogurt and vanilla extract; beat until blended.

Step 4

Add flour mixture to banana mixture, mix gently with a wooden spoon just until moist. Drop peanut butter on top of batter, swirl gently.

Step 5

Spoon batter into the prepared loaf pan. Bake for 50-55 minutes or until a wooden pick inserted in the center comes out clean.Cool 10 minutes in pan on wire rack; remove from pan and let cool completely on wire rack.