Prepare Your Tin: Lightly grease a 20cm (8-inch) square baking tin with butter. Line it with baking parchment, leaving some overhang on two sides to act as 'handles' for easy removal later.

Melt the Butter: In a large, heavy-based saucepan or pot, melt the 60g of unsalted butter over a low heat. Don't let it brown, just melt it gently.

Melt the Marshmallows: Once the butter has melted, add the 300g of marshmallows to the pan. Continue to stir over a low heat for 5-7 minutes until the marshmallows are completely melted and the mixture is smooth and glossy. Keeping the heat low is vital to prevent the sugar from seizing up.

Add Flavourings: Remove the saucepan from the heat. Immediately stir in the vanilla extract and the sea salt until fully combined.

Combine with Cereal: Add the Rice Krispies to the saucepan. Using a spatula, gently fold the cereal into the marshmallow mixture until every grain is evenly coated. Work quickly but carefully to avoid crushing the cereal.

Press into the Tin: Tip the mixture into your prepared tin. What works best for me is to use another piece of baking parchment or a lightly greased spatula to gently press the mixture evenly into the corners. Don't press down too hard, as this can make the treats dense.

Melt the Chocolate: For the topping, place the chopped milk and dark chocolate into a heatproof bowl. You can melt it in the microwave in 30-second bursts, stirring in between, or set the bowl over a saucepan of gently simmering water (a bain-marie), ensuring the bottom of the bowl doesn't touch the water. A guide on how to melt chocolate correctly can be very helpful here.

Add the Topping: Once the chocolate is smooth and melted, stir in the optional vegetable oil for extra shine. Pour the melted chocolate over the Rice Krispie base in the tin. Use an offset spatula or the back of a spoon to spread it into an even layer.

Chill Until Set: Place the tin in the refrigerator and chill for at least 2 hours, or until the chocolate topping is completely firm.

Slice and Serve: Once set, use the parchment paper handles to lift the entire block out of the tin. Place it on a cutting board and use a large, sharp knife to cut it into 16 squares. Let them sit at room temperature for 10-15 minutes before serving for the best chewy texture.