Preheat oven to 350°F (175°C) and grease a 12-cup muffin tin or line with paper liners.

In a large mixing bowl, whisk together mashed bananas, Greek yogurt, maple syrup, egg, and vanilla extract until well combined.

In a separate bowl, whisk together the whole wheat pastry flour, baking soda, and salt.

Gently fold the dry ingredients into the wet ingredients until just combined.

Divide the batter into two equal portions in separate bowls.

Stir the unsweetened cocoa powder into one of the portions until smooth.

Spoon alternating dollops of the plain banana batter and the chocolate batter into the prepared muffin cups.

Place 1 teaspoon of warmed peanut butter on top of each muffin and use a toothpick to swirl the batters and peanut butter together.

Bake for 18 to 22 minutes or until a toothpick inserted into the center comes out clean.

Cool in the pan for 5 minutes before transferring to a wire rack.