Cream the Butter and sugars together until light and fluffy, then add the egg, yolk and vanilla. Mix until well combined.
Add the sifted dry ingredients and mix until just combined. Stir in the chocolate chips.
Scoop a 1/4 cup of dough (or heaped tablespoon) and form in to a rough ball, then place on a lined baking tray. Chill the tray of cookie dough balls in the refrigerator for approx 30mins. Preheat oven to 170C.
Bake until the cookies are light golden brown and the outer edges start to harden yet the centres are still soft and puffy, 15-18mins.
Cool completely before storing in an airtight container. Makes 18 large cookies.