Line an 8-inch square baking pan with parchment paper or aluminum foil. Set aside.
Combine the sliced butter and peanut butter in a large saucepan and place it over medium heat, stirring occasionally, until the butter and peanut butter are melted and the mixture is completely smooth.
Remove from the heat and stir in the vanilla extract and salt. Whisk in 1 cup (120 grams) of powdered sugar at a time until well combined, then stir in the last ½ cup (60 grams) of powdered sugar until fully combined and smooth.
Scoop the mixture into the prepared baking pan and firmly press it down into one even layer. Smooth out the top, then transfer it to the refrigerator to chill for at least 2 hours.
Once the fudge is firm, slice it into pieces, and enjoy!