Preheat the oven to 350, and line a small cookie sheet with parchment paper.
In a small bowl, combine the melted butter and brown sugar. Mix the butter and brown sugar together with a small spatula.
Next, add the egg yolk and vanilla extract and mix well.
Then, sprinkle the flour and salt evenly over the bowl, and stir just until combined.
Finally, sprinkle in the chocolate chips.
Scoop the cookie dough onto the cookie sheet and try to mound it up a bit. Place a few more chocolate chips on top.
Bake for 16-18 minutes. The cookie is done when the edges are turning golden brown, but the center may stay a bit gooey and soft. Let the cookie cool completely on the pan so that it sets.
