Prep the pan. Line a 9-inch square pan with parchment, leaving overhang on two sides to lift the fudge out later. Lightly grease the parchment if your peanut butter is extra sticky.

Make the peanut butter base. In a large bowl, beat the softened butter and peanut butter together until smooth and creamy. Stir in the vanilla and salt.

Add the powdered sugar. Mix in the powdered sugar 1 cup at a time. The mixture will thicken into a soft, dough-like fudge. If it seems crumbly, knead briefly with clean hands until smooth.

Press into pan. Transfer the peanut butter mixture to the prepared pan. Press it into an even layer using a spatula or the bottom of a measuring cup. Smooth the top.

Melt the chocolate. In a microwave-safe bowl, combine the chocolate chips and 2 tablespoons butter. Microwave in 20–30 second bursts, stirring between each, until just melted and glossy. You can also melt using a double boiler on the stove.

Top the fudge. Pour the melted chocolate over the peanut butter layer. Tilt the pan to spread, or use an offset spatula to smooth into a thin, even coating. Sprinkle a pinch of flaky sea salt if using.

Chill to set. Refrigerate for 1–2 hours, or until the chocolate is firm to the touch and the fudge lifts cleanly from the pan.

Slice and serve. Lift the fudge out using the parchment overhang. Place on a cutting board and slice into small squares with a sharp knife. For clean edges, warm the knife under hot water and wipe between cuts.