Line a 6 inch square cake pan with parchment paper. (I cut mine 6”x10” so two sides are longer and stick up slightly)
Add unsalted butter and peanut butter to a medium sauce pan over medium heat and bring to a rolling boil, stirring occasionally.
Remove from heat and add in powdered sugar, salt and vanilla, stirring until completely mixed in.
Transfer to prepared cake pan and smooth top flat with a spatula.
Cool at room temperature, about an hour, until set.
When fudge has set, run a knife gently around edges and invert it onto a cutting board.
Remove parchment paper and cut into 9, 12, or 16 equal squares.
Video
Notes
Store in an airtight container at room temperature for about 1 week (out of sunlight) or in the fridge for 2 to 3 weeks refrigerated. To freeze, wrap chilled fudge in plastic wrap and then transfer to a freezer storage bag. Freeze for up to a month and thaw in the fridge.
Nutrition is based on 16 pieces. Nutrition information is a guideline only, is calculated automatically by third party software, and absolute accuracy is not guaranteed.