In a small saucepan, melt the butter. Set aside to cool.
While the butter is cooling, mix together the flour, baking soda and salt.
In a large mixing bowl, mix together cooled butter and both sugars. Stir in the egg and egg yolk until blended. Stir in the vanilla, then add the flour mixture and stir until blended. Stir in the chocolate.
Divide batter into 14 to 16 (depending on how big you want the cookies) equal sections and shape into balls. Chill balls overnight or for several hours.
When ready to bake, arrange balls of dough 3 inches apart on a foil lined baking sheets. Bake in a preheated 325 degree oven for 13 to 15 minutes or until cookies appear set. As soon as you pull them out of the oven, tap the edges inward with a spatula so make the cookies round (they do spread a bit).