Method
Cream the melted butter and both sugars together in a bowl until combined and slightly creamy.
Whisk in the egg, egg yolk and vanilla until the mixture is light and creamy.
In a separate bowl, mix the flour, baking soda and cornstarch until evenly combined.
Add the dry ingredients to the wet ingredients and mix until just combined. Fold in the chocolate chips.
Scoop out about 1/4 cup of dough, roll into a ball and place on a parchment-lined baking sheet.
Place the sheet of cookie dough balls in the freezer for 15 minutes to firm up.
Preheat the oven to 325°F (about 160°C).
Arrange cookies on the baking sheet spaced about 3 inches apart.
Bake for 12–15 minutes, until the edges are golden but the centers still look slightly under-baked.
Cool on a wire rack for at least 5 minutes before serving.
Notes
Milk chocolate chips were used here; these are large, chewy cookies. Recipe originally borrowed from 'Mom on Timeout'. Many double the batch.