Line a 8x4 loaf pan with parchment paper, set aside.
Heat a medium saucepan over medium heat. Melt your butter, then add in your brown sugar replacement. Stir continuously until sugar dissolves and it begins to simmer and a thick brown caramel begins to form. Usually 5-8 minutes.
When the sauce is thickened, remove from the heat and stir in the peanut butter and vanilla extract until smooth.
Carefully pour your fudge into the prepared loaf pan.
Freeze 30 minutes or refrigerate for 1 hour. Or until fully set.