This recipe involves 100g of filling however, I like to split depending on what I'd like.
For example, if you would like hazelnut and chocolate chip cookies, use 50g of each.
If you would like 3 ingredients, use 33g of each.
The number should always equal 100g as to not overload the cookie.
I wouldn't personally do anymore than 3.

Preheat oven to 190c/170c Fan/Gas Mark 6.
Line 2-3 baking trays with greaseproof paper or oil them.

Put butter and sugars into large mixing bowl, beat until evenly blended.
Beat the egg in a different bowl/cup and add vanilla extract then slowly add this into the butter/sugar mix - beating well.
Mix in flour then stir in (with a spoon) the chocolate chips (ETC).

Spoon + roll mixture into balls (weight 30g per ball - check cookie size chart tip) and put onto baking trays 2 inches apart from all sides.
Bake in oven for 8-10 mins or until golden brown.
Leave cookies to cool on tray then move onto a cooling rack.

Fan oven tip.