Line your tray: Line a large baking tray with parchment paper. You'll want it ready before you start cooking — the mixture sets fast.

Melt the base: In a medium saucepan over medium heat, combine the caster sugar, milk, butter, and cocoa powder. Stir continuously until the butter has melted and the mixture is smooth and glossy — you'll see it turn a deep, rich brown, and you'll smell the cocoa blooming.

Bring to a boil: Increase the heat to medium-high and bring the mixture to a full rolling boil. Let it boil for exactly 1 minute without stirring. You'll see large bubbles breaking the surface, and the mixture will thicken slightly — this is crucial for the cookies to set properly.

Remove from heat: Take the saucepan off the heat immediately. Stir in the peanut butter and vanilla extract until fully combined. The mixture should be smooth and silky, with a glossy sheen.

Add the dry ingredients: Fold in the porridge oats, desiccated coconut (if using), and a pinch of sea salt. Stir until every oat is coated — the mixture will look thick and lumpy, and you'll smell the warm peanut butter and oats together.

Scoop and shape: Working quickly while the mixture is still warm, use a tablespoon or small cookie scoop to drop rounded portions onto the lined tray. If the mixture starts to stiffen, you can press each cookie gently with the back of a spoon to flatten it slightly.

Chill to set: Place the tray in the refrigerator for at least 30 minutes. The cookies are ready when they are firm to the touch and no longer sticky. You'll see they've set into solid, slightly shiny discs.

Serve or store: Once set, transfer the cookies to a wire cooling rack if you like, or simply enjoy them straight from the fridge. They will continue to firm up as they cool completely.