Prepare the baking dish. Line a 20cm square baking dish with two long strips of aluminium foil, leaving an overhang on two opposite sides. This creates a sling for lifting the fudge out later. Lightly grease the foil with a little butter. You’ll hear the foil crinkle as you press it into the corners—make sure it’s smooth, as any creases will mark the fudge’s surface.

Make the peanut butter base. In a large mixing bowl, beat the softened butter and peanut butter together with an electric mixer on medium speed until the mixture is pale and fluffy. This should take about 2 minutes. The colour will lighten from a deep tan to a soft caramel, and it will smell intensely nutty and rich.

Add the sugar. With the mixer on low, gradually add the sifted icing sugar and the vanilla extract. The mixture will first look like dry crumbs, then slowly come together into a thick, cohesive dough. Stop mixing once it’s fully combined—the texture should feel like soft, pliable playdough. Scrape down the sides to ensure no streaks of butter remain.

Press the base into the dish. Transfer the peanut butter mixture into the prepared dish. Using the back of a metal spoon or your fingertips (lightly dampened to prevent sticking), press it into an even, compact layer. The surface should be perfectly flat and level. You’ll feel the resistance as it compresses. Pop the dish into the fridge to chill for at least 30 minutes, or until the base is firm to the touch.

Prepare the chocolate topping. While the base chills, place the chopped dark chocolate, milk chocolate, and double cream in a heatproof bowl set over a pan of barely simmering water. The water must not touch the base of the bowl. Stir gently with a rubber spatula until the chocolate is completely melted and the mixture is smooth and glossy. If using coconut oil, stir it in now. The mixture will be thick and should fall from the spatula in a silky ribbon.

Pour and set the chocolate layer. Remove the chilled peanut butter base from the fridge. Pour the melted chocolate over the top. Using an offset spatula, spread it evenly to the very edges, ensuring it covers the entire surface. Tilt the dish gently to help it settle. The chocolate should look like a dark, polished mirror. Tap the dish lightly on the counter to release any air bubbles.

Final chill. Place the dish back in the fridge for at least 2.5 hours, or until the chocolate is completely set and firm. When you press the surface, it should feel solid and not leave a fingerprint.

Slice and serve. Using the foil sling, lift the entire block of fudge out of the dish. Place it on a cutting board. Using a sharp, large knife, slice into even squares. For the cleanest cuts, wipe the knife clean with a hot, damp cloth between cuts. The top layer will crack with a satisfying snap, revealing the creamy, pale centre beneath.