Line an 8×8-inch square baking pan with parchment paper or aluminum foil. Leave a little bit of overhang on the sides so you can easily lift the fudge out later. Lightly grease the paper with a tiny bit of butter or non-stick spray to ensure a clean release.

In a large mixing bowl, combine the softened butter and the creamy peanut butter. Use a sturdy spatula or an electric hand mixer to blend them until the mixture is completely smooth and uniform in color. Stir in the vanilla extract and salt until well incorporated.

Gradually add the powdered sugar one cup at a time. Mix thoroughly after each addition. The mixture will become very thick and slightly stiff as you add more sugar. Continue mixing until no white streaks remain and the “dough” is smooth and pliable.

Transfer the peanut butter mixture into your prepared pan. Use the back of a spoon or an offset spatula to press the fudge firmly into the corners and smooth out the top. Place the pan in the refrigerator for at least two hours, or until the fudge is firm to the touch. Once set, lift it out by the parchment paper, slice into squares, and serve.