In a mixing bowl, add in the butter and peanut butter. Microwave for 45 seconds and stir. If there are still lumps, microwave an additional 30 seconds. Stir until smooth.
Add in the maple syrup and continue to stir until incorporated.
Add in all of the powdered sugar and stir until it's combined. It will become thick and hard to stir.
Place into the prepared pan and spread out.
Place a piece of parchment paper or wax paper on top of the fudge. Smooth out using your hand over top of the wax paper.
Place in the freezer for 45 minutes or the fridge for at least 2 hours until the fudge is cooled. Cut the fudge and serve.
I keep this fudge in the fridge to keep it hard. If it comes to room temperature, it will be soft.