In the bowl of a stand mixer, beat together butter, brown sugar and granulated sugar . Add the egg and vanilla extract on low speed and beat until well blended.
Sift flour, baking soda and salt into the creamed butter.
Fold in the snickers bar chunks and chocolate chunks.
Cover the cookie dough and chill in the fridge for at least 2 hours or overnight. ( The cookie dough will be quite sticky at this stage to roll)
Take the dough out of the fridge and leave to soften to room temperature for 10 minutes.
Pre-heat oven to 160°C Fan/180°C/350°F. Line 2 baking tray with parchment paper.
Scoop two tablespoons each of the cookie batter and roll into balls. Place cookie balls on the baking tray at least 2 inches apart. If desired, press a few snickers chunks on top of the cookie dough balls.
Bake for 10 minutes. Remove from oven and place on cooling rack. The centre of cookies may look slightly under baked. Leave to cool for at least 10 minutes before serving.