Place the unsalted butter in a microwave safe bowl. Microwave for 20-30 seconds until the butter has melted.
Add the two sugars, vanilla extract, whole egg and egg yolk into the melted butter and mix together until fully combined.
Now fold in the flour and salt. The mixture will come together to form a cookie dough.
Chop up the Mars Bars into bite-sized pieces and add them into the cookie dough (save a handful for later) and fold into the cookie dough.
Cover the bowl with clingfilm and refrigerate overnight, or for at least 4 hours.
Roll and bake the cookies
Preheat the oven to 180°C (fan oven) and line 2 large, flat baking trays with greaseproof paper.
Take the cookie dough out of the fridge and leave to soften at room temperature for 15 minutes.
Scoop up a heaped tablespoon of cookie dough and use your hands to roll it into a ball. Place on the baking tray and do not flatten. Repeat this step until all the cookie dough has been used up. Remember to leave roughly 8-10cm between each ball so they have enough room to spread out.
Bake for 10-12 minutes. The cookies should be golden on the outside but the middle still pale and soft. Don't worry if the middle still looks under-baked, this is how they are meant to be and the cookies will carry on baking as they cool down.
As soon as the cookies are out of the oven, push a few leftover Mars Bar chunks into the top of each cookie.
Leave on the baking tray for 15 minutes to cool down and firm up. This will change the cookie texture so try to resist the temptation to tuck in straight away.
Store your Mars Bar Cookies at room temp, in an airtight container, for up to 4 days.
Keyword Eggs, Flour, Light brown sugar, Mars bars, Self-raising flour