In a heat-safe mixing bowl suitable for microwave use, combine the unsalted butter and peanut butter. Heat on high power for 2 minutes. Remove, stir to blend, and microwave again for an additional 1 to 2 minutes until the mixture is thoroughly melted and homogeneous.

Add the vanilla extract to the warm peanut butter mixture and stir well to incorporate.

Gradually fold in the powdered sugar, working in increments. Continue stirring until a dense, uniform dough-like consistency is achieved. Expect the mixture to become quite thick.

Transfer the prepared fudge mixture into a square 8-by-8-inch pan lined with parchment paper or lightly greased. Press evenly into the dish, smoothing the surface as needed.

Place the pan in the refrigerator and allow the fudge to firm up for a minimum of 2 hours.

Once fully set, slice the chilled fudge into neat squares and serve.