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Beat the butter, sugar and syrup together in a food mixer with a paddle attachment until light and fluffy.
Beat the egg in a separate bowl and mix in the vanilla paste then mix into the beaten butter and sugar.
Fold in the remaining ingredients except the marshmallow pieces. Mix well to form a cookie dough.
Split the dough into 40g balls and place them on a Pyrex glass baking tray lined with baking parchment. Flatten the dough balls slightly leaving enough room for them to spread.
Chill the cookie dough for at least an hour before baking in a preheated 180°C oven.