Combine the peanut butter, butter, and vanilla in a microwave-safe bowl. Microwave for 90 seconds until melted. Stir until smooth, then gradually add powdered sugar, mixing well after each addition.

Line an 8-inch square pan with parchment paper or lightly butter it. Pour the fudge mixture into the pan and spread evenly.

Chill in the refrigerator for at least 4 hours or until firm. Slice into squares and serve.