Add in the egg yolk, the brown sugar, and the vanilla if you're using some and stir until incorporated, Add in the flour and stir one more time just until the flour disappears. At this point you can add the chocolate chips and gently stir to get them incorporated to skip this step.
1 egg yolk, 1 tablespoon brown sugar & 1 teaspoon brown sugar, ⅛ teaspoon vanilla, 2 tablespoon flour, 1 tablespoon chocolate chips