Prepare the pan. Line a 23x33cm baking tray with parchment paper, leaving an overhang on two sides. Lightly butter the paper and the exposed sides of the tray. This step is crucial; the mixture will stick to anything ungreased.

Melt the butter. Place the 200g of butter into a large saucepan over a low heat. Watch it gently foam and bubble. When the butter has completely liquefied and the foam subsides, the colour should be a pale, sunny yellow. You will smell a nutty, rich aroma.

Melt the marshmallows. Add the entire 500g of marshmallows to the melted butter. Stir constantly with a silicone spatula. The marshmallows will first turn glossy and slick, then begin to slump and stretch. After about 3–4 minutes, the mixture will become a smooth, thick, ivory-coloured sauce. The sound will shift from a soft squelch to a quiet, even bubble.

Add the vanilla and salt. Remove the pan from the heat. Stir in the teaspoon of vanilla extract and the pinch of sea salt. The steam will carry the scent of vanilla upward. Mix for 15 seconds until everything is homogenous.

Incorporate the cereal. Pour the 300g of Rice Krispies into the pan. Fold the cereal into the marshmallow mixture using a gentle, sweeping motion. You want every grain to be coated. The texture will become stiff and slightly resistant. You will hear the distinct crackle and pop of the cereal as it meets the warm, sticky coating.

Transfer to the pan. Tip the mixture into the prepared baking tray. Do not press it down with force. Instead, use a piece of buttered parchment paper or a lightly oiled spatula to gently coax it into the corners. The top should be even but not compacted. If you press too hard, the treats will turn out dense and hard.

Cool completely. Allow the tray to sit at room temperature for at least 30 minutes. Do not refrigerate to speed this up; it can make the treats tough. The surface should feel set and firm to the touch, and the edges should have pulled away slightly from the pan.

Slice and serve. Lift the block out using the parchment overhang. Place it on a cutting board. Use a large, sharp chef's knife to cut into 16 squares. The blade will glide through with a clean, satisfying snap. Serve at room temperature.