Melt your butter in the microwave for 30 seconds in a large microwave-safe mixing bowl.
Once the butter is melted, mix in the heavy cream, brown sweetener, and vanilla extract. Place the bowl back in the microwave for 3 minutes to condense the cream. The contents of the bowl will bubble up so be sure to use a large bowl to prevent it from bubbling over.
Next, mix the peanut butter and coconut flour into the fudge. Once it has been mixed in completely, pour the fudge into a small bread pan lined with parchment paper or into a silicone ice-cure tray.
Place the fudge in the freezer and allow it to harden completely (at least 2-3 hours) before serving. Fudge should be stored in the refrigerator or freezer in an air-tight container and kept chilled until it is ready to serve.