Prep the pan: Line an 8x8-inch square pan with parchment paper, leaving overhang on two sides for easy lifting. Lightly grease the parchment if you like ultra-clean cuts.

Melt the base: In a medium saucepan over low heat, add 1 cup (225 g) unsalted butter and 1 1/2 cups (about 380–400 g) creamy peanut butter. Stir often until completely smooth and just starting to bubble around the edges. Do not boil.

Flavor it: Remove from heat. Stir in 1 1/2 teaspoons vanilla extract and a pinch (about 1/4 teaspoon) of fine salt. Mix until glossy.

Sweeten and thicken: Gradually sift in 3 1/2 to 4 cups powdered sugar, about 1 cup at a time, stirring well after each addition. Stop when the mixture is thick, smooth, and holds soft ridges. It should be spreadable, not runny or crumbly.

Optional chocolate topping: Melt 1 cup semi-sweet chocolate chips with 1 tablespoon coconut oil or butter in the microwave in 20–30 second bursts, stirring between each, until smooth. Set aside briefly to cool slightly.

Assemble: Scrape the peanut butter mixture into the lined pan. Smooth the top with an offset spatula. If using chocolate, pour it over and spread evenly. Sprinkle optional chopped peanuts or flaky salt.

Set and slice: Chill for 1–2 hours, or until firm. Lift the slab out by the parchment, set on a cutting board, and slice into small squares with a sharp knife. Wipe the blade between cuts for neat edges.