Preheat and Prepare: First, preheat your oven to 180°C (160°C fan) and line two large baking trays with parchment paper. This prevents the cookies from sticking and makes clean-up much easier.

Cream Butter and Sugars: In a large bowl, using a stand mixer or an electric hand mixer, beat the softened butter with the soft light brown sugar and caster sugar for 2-3 minutes until the mixture is light, pale, and fluffy.

Add Wet Ingredients: Add the egg and vanilla extract to the butter mixture and beat again for another minute until everything is well combined. Scrape down the sides of the bowl to ensure it's all incorporated.

Combine Dry Ingredients: In a separate bowl, whisk together the plain flour, bicarbonate of soda, and salt. Gradually add this dry mixture to the wet ingredients, mixing on a low speed until just combined. Be careful not to overmix at this stage.

Fold in the Goodies: Gently fold in the chopped milk chocolate and the crushed digestive biscuits using a spatula. What works best for me is to reserve a small handful of each to press onto the tops of the cookie dough balls for a more professional look.

Form the Cookies: Scoop out tablespoon-sized balls of dough (about 40g each) and place them on your prepared baking trays, leaving a good amount of space between each one as they will spread. I usually fit about 9 cookies per tray.

First Bake: Place the trays in the preheated oven and bake for 8 minutes. The cookies will be slightly puffed up and just starting to turn golden at the edges.

Add the Marshmallow: Carefully remove the trays from the oven. Gently press one large marshmallow into the centre of each partially baked cookie.

Final Bake: Return the trays to the oven and continue baking for another 3-4 minutes, or until the edges of the cookies are golden brown and the marshmallows are puffed up and lightly toasted. Keep a close eye on them during this stage, as marshmallows can catch quickly!

Cooling is Key: Let the cookies cool on the baking tray for 5-10 minutes before transferring them to a wire cooling rack to cool completely. The marshmallows will deflate as they cool, which is perfectly normal. This resting time allows the cookies to set properly.