Line an 8-by-8-inch pan with parchment, leaving overhang on two sides.

Combine butter, sugar, cocoa and milk in a saucepan and bring to a full rolling boil.

Boil for exactly 1 minute, stirring constantly, then remove from heat.

Stir in peanut butter, vanilla and salt until smooth, then fold in oats.

Press the warm mixture firmly and evenly into the pan.

Refrigerate for 2 hours, lift out by the parchment and cut into 16 squares.