Prepare Your Tin: Lightly grease a 20cm x 20cm (8-inch x 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.

Brown the Butter: In a large saucepan or Dutch oven, melt the 120g of unsalted butter over a medium-low heat. Continue to cook, swirling the pan occasionally, for about 5-8 minutes. The butter will foam up, then subside. Look for little brown specks to appear at the bottom of the pan and listen for the sizzling to quiet down. It should smell wonderfully nutty. For a deep dive into the technique, Serious Eats has a great guide on the science of browning butter.

Melt the Marshmallows: Once the butter is a lovely amber colour, turn the heat down to low and add the 300g of mini marshmallows all at once. Stir continuously with a spatula until the marshmallows are completely melted and the mixture is smooth.

Add Flavour: Remove the pan from the heat. Immediately stir in the vanilla extract and the fine sea salt. The mixture might bubble up for a second – this is normal.

Combine with Cereal: Add the puffed rice cereal to the pan. Using a silicone spatula, gently fold everything together until the cereal is evenly coated in the marshmallow mixture. Work quickly here as it will start to set. What works best for me is using a folding motion from the bottom up to avoid crushing the cereal.

Press into the Tin: Scrape the mixture into your prepared tin. Lightly grease your spatula or your fingertips with a little butter or water and gently press the mixture into an even layer. Don't compact it too firmly, or the treats will be hard.

Finish and Set: Sprinkle the top evenly with the flaky sea salt. Leave the tin on the counter at room temperature for at least 1 hour to set completely. You can speed this up in the fridge for 30 minutes, but I find the texture is best when set at room temperature.

Slice and Serve: Once firm, use the parchment handles to lift the slab out of the tin. Place it on a cutting board and slice into 16 squares with a sharp knife. Enjoy!