Line an 8x8 inch pan with parchment paper. Set aside.

Place the peanut butter and butter in a large (3-quart) microwave-safe bowl. Cover loosely with plastic wrap and microwave on full power for 2 minutes. Stir the mixture with a wooden spoon or rubber spatula, re-cover, and microwave for an additional 2 minutes.

Remove the bowl from the microwave, stir again, and add the vanilla, salt, and powdered sugar. Stir until all the powdered sugar is combined and no lumps remain. The mixture will be very thick.

Press the mixture into the prepared pan evenly. Cover the fudge with the remaining parchment or cling wrap and refrigerate for 2 hours, or until cooled and set.

Cut into 1-inch pieces.