Line a 9x9 inch brownie pan with parchment paper and lightly grease it with coconut oil or melted butter.
In a large mixing bowl, combine peanut butter with maple syrup and stir until the mixture reaches a smooth consistency. Add oat flour and optionally protein powder, then stir to combine.
Transfer the peanut butter dough to the prepared baking pan and spread it out with a spatula or with the help of your hands.
Place dark chocolate chunks (or chocolate chips) in a microwave-safe bowl with 1 tablespoon of peanut butter. Microwave in 30-second intervals, stirring in between, until the chocolate has melted completely.
Pour melted chocolate over the dough and spread evenly.
Refrigerate for at least 1 to 2 hours, or until the chocolate has hardened. Slice into 16 bars, then serve immediately or place in an airtight container and store in the fridge for a maximum of 2 weeks.