Prepare your workspace: Line a large baking tray with parchment paper. I always do this first because the mixture sets quickly once it's off the heat.

Melt the base: In a medium saucepan over medium heat, combine the caster sugar, milk, butter, and cocoa powder. Stir constantly with a wooden spoon until the butter has fully melted and the mixture is smooth and glossy, about 3 minutes. You'll know it's ready when the surface looks shiny and the sugar has completely dissolved—rub a tiny bit between your fingers; it should feel smooth, not gritty.

Bring to a gentle boil: Increase the heat slightly and let the mixture come to a gentle boil. Allow it to bubble for exactly 1 minute, stirring occasionally. The mixture will darken slightly and smell deeply chocolaty. Do not skip this boiling step, as it helps the cookies set properly.

Remove from heat and add peanut butter: Take the saucepan off the heat immediately. Stir in the peanut butter and vanilla extract until fully incorporated. The mixture will become thick and creamy, with a rich nutty aroma filling your kitchen.

Fold in the dry ingredients: Add the rolled oats, desiccated coconut if using, and the pinch of salt. Stir vigorously until every oat is coated in the chocolate mixture. You should see no dry patches of oats, and the mixture should hold together when pressed between your fingers.

Scoop and shape: Working quickly while the mixture is still warm, use a tablespoon or a small cookie scoop to portion out the dough. Drop each scoop onto the prepared baking tray, leaving about 2cm between each cookie. Gently press down on each one with the back of the spoon to flatten slightly into a cookie shape.

Chill until firm: Place the tray in the refrigerator for at least 30 minutes, or pop it in the freezer for 5 to 10 minutes if you're in a hurry. The cookies are ready when they feel firm to the touch and can be lifted off the parchment without bending or breaking.

Serve and store: Transfer the chilled cookies to a serving plate or an airtight container. They will keep at room temperature for up to 5 days, but I find they rarely last that long in my house.