Prepare your pan. Line a 20cm x 20cm square baking tin with a long sheet of parchment paper, leaving some overhang on two sides. This creates your own lifting handles for later. Lightly grease the paper with a smear of butter or a spritz of cooking spray.

Melt the butter. Place the butter in a large, heavy-bottomed saucepan over medium-low heat. Watch as the butter begins to foam, then turns a pale, nutty brown. You will smell a warm, toasty aroma when it is ready — this takes about 3 to 4 minutes. Do not let it go dark.

Add the marshmallows. Pour all the mini marshmallows into the saucepan. Stir constantly with a silicone spatula. The marshmallows will begin to slouch, then collapse into a creamy, pillowy mass. The mixture will be glossy and stretchy, and it should feel silky when you lift the spatula. Keep stirring until no white streaks remain.

Remove from heat and add flavour. Take the pan off the burner entirely. Stir in the vanilla extract and the flaky salt. If you are adding nut butter, stir it in now until the paste is fully incorporated and the colour is uniform.

Fold in the cereal. Pour the crispy rice cereal into the saucepan. Using a gentle folding motion — like you are turning a book page — combine the cereal with the marshmallow mixture. Fold until every grain is coated and the mixture looks like a speckled, sticky dough. The sound will change from a slurp to a soft, crackly rustle.

Press into the pan. Tip the mixture into the lined tin. Using a piece of greased parchment paper or a silicone spatula, press the mixture firmly and evenly into the corners. Press down with steady, even pressure. Do not flatten it too hard, or the treats will become dense and hard. Aim for an even, dimpled surface.

Cool and set. Let the tin sit at room temperature for at least 30 minutes. The treats will firm up as they cool. The surface should feel dry and springy to the touch, and the sides will pull away slightly from the parchment.

Slice and serve. Lift the block out using the parchment overhang. Place it on a cutting board. Using a sharp, serrated knife, cut into 12 even squares. The knife should glide through with a clean, crisp snap. For the neatest edges, wipe the blade clean between cuts.