Line an 8x8-inch pan with parchment paper, leaving overhang on two sides for easy removal.

Combine the peanut butter and butter in a large microwave-safe bowl. Microwave in 30-second intervals, stirring between each, until melted and smooth (about 1.5 minutes total).

Stir in the vanilla extract.

Gradually stir in the powdered sugar, one cup at a time, until the mixture is thick and smooth.

Pour into the prepared pan and spread into an even layer. Smooth the top with a spatula.

Refrigerate for at least 1 hour until firm, then cut into squares.