Preheat the oven to 350 degrees F.

Sift the flour, cocoa powder, baking soda and salt into a medium bowl and set aside. In a separate large bowl cream the butter and sugar together. Stir the eggs, mashed bananas and vanilla together in a large measuring cup. Gradually pour the banana mixture into the butter and sugar mixture until all ingredients are combined well.

With a wooden spoon or rubber spatula, slowly stir the flour mixture into the liquid mixture until just incorporated. Fold in the chocolate chips.

Line a 12 cup muffin pan with paper liners or grease the pan if not lining it. Pour enough muffin batter into each muffin cup until it covers the bottom. Then scoop a heaping tablespoon of peanut butter into each muffin cup. Pour the rest of the batter over the peanut butter dollops. Bake for 20-25 minutes or until a toothpick comes out clean without any batter on it. (Peanut butter will come out on the toothpick. That is OK as long as the batter is set.)