Cut a 22 piece of parchment paper. Fold the long sides of the sheet in to make an 8 wide strip. Butter an 8 x8 square dish. Place the parchment paper into the dish with the folds underneath, creasing the corners well to make a clean edge. Butter the parchment lightly and set aside.
Melt 4 tbsp butter in a medium saucepan over medium heat. When fully melted, add vanilla, brown sugar, white sugar and cream. Stir well. Bring to a full rolling boil. Once it boils, stir constantly for exactly two minutes (use a timer). Add the peanut butter and stir until melted and completely smooth. Remove from heat and let cool exactly 5 minutes (use a timer). It will form a skin; that's okay.
Scrape the mixture into the bowl of a stand mixer fitted with the paddle attachment. Start on medium-low (speed 2), and add the pre-sifted powdered sugar one large spoonful at a time. When it is all incorporated, the fudge should look slightly curdled.
Turn the fudge into the prepared pan and spread evenly. Sprinkle chopped peanuts evenly over the top. Fold over the ends of the parchment, then use the bottom of a measuring cup or glass to press the peanuts into the fudge and form a layer of even thickness. Unfold the parchment and let cool to room temperature, at least 2 hours.
When cool, lift the ends of the parchment straight up to remove the fudge from the pan. Using a long knife, cut the fudge into 64 pieces (8 slices one way, then 8 the other way). Store in an airtight container at room temperature for up to 3 days, or in the refrigerator for up to a week. For best flavor, return to room temperature before eating.