Line an 8x8-inch baking pan with parchment paper, leaving overhang on two sides for easy removal later.
In a large microwave-safe bowl, melt the butter in 30-second intervals until fully liquid. Add the peanut butter and stir until smooth and well combined.
Stir in the vanilla extract, then add the sifted powdered sugar and graham cracker crumbs. Mix with a sturdy spoon until the mixture is thick, smooth, and no dry pockets remain.
Transfer the fudge mixture to the prepared pan. Use the back of a spoon or a spatula to press it down firmly into an even layer, smoothing the top completely.
Refrigerate for at least 2 hours until firm to the touch. The fudge should feel solid but not rock-hard when gently pressed.
Lift the fudge out using the parchment overhang and place on a cutting board. Use a sharp knife to cut into 36 one-inch squares, wiping the blade between cuts for clean edges.
