½ cup unsalted butter 2 ¼ cups light brown sugar ½ cup whole milk ¾ cup smooth peanut butter 1 teaspoon vanilla extract 3 ½ cups icing sugar
Stir in peanut butter and vanilla extract until combined.
Add icing sugar and beat until smooth.
Spread fudge into prepared baking dish, and smooth the top. Let it sit at room temperature until it cools and sets, about 2 hours.
Store in an airtight container in the refrigerator for up to 2 weeks.