Sift sugar through a fine-mesh strainer into a large bowl.
Heat butter and peanut butter in a saucepan over medium heat. Whisk to combine and cook until bubbles appear on the edges, 3 to 5 minutes. Stir in salt and vanilla extract. Pour over powdered sugar in the bowl. Mix together using a spatula until smooth.
Spread mixture into an 8×8-inch baking pan lined with plastic wrap. Wrap fully and refrigerate until firm enough to cut but not too hard, 1 to 3 hours.
Unwrap fudge and cut into small cubes. Let warm up on the counter before cutting if needed.
This technique will work with other nut butters, smooth or chunky, like almond or cashew.
Use 1/4 teaspoon fine salt instead of kosher if desired.
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