In a large mixing bowl, whisk together the melted butter, oil, brown sugar, and granulated sugar until well combined. Beat in the eggs one at a time, followed by the vanilla extract and the mashed ripe bananas.

In a separate small bowl, whisk together the flour, cocoa powder, baking soda, and salt.

Gently fold the dry ingredients into the wet ingredients until just combined. Fold in the chocolate chips, reserving some for topping.

Scoop about 1/3 of the banana bread batter out and place it in a small bowl. Stir the creamy peanut butter into this portion until fully combined.

Pour about half of the plain chocolate batter into the prepared slow cooker. Dollop half of the peanut butter mixture randomly over the top. Swirl gently with a butter knife or skewer. Repeat with the remaining chocolate batter and peanut butter mixture. Top with additional chocolate chips.

Cover and cook on HIGH for 2-3 hours, or until a toothpick inserted into the center comes out mostly clean. Note: Cooking times may vary depending on your slow cooker; start checking at 2 hours.

Allow the bread to cool in the slow cooker with the lid slightly ajar for 30 minutes, then carefully remove and let cool completely on a wire rack before slicing.