Combine melted butter and brown sugar in the bowl of a stand mixer.

Beat until mixture is thick and looks like thin peanut butter, scraping bowl as necessary.

Add vanilla and egg plus yolk and beat in until mixed well.

Slowly add flour mixed with salt and beat on low speed until almost mixed.

Add chocolate and stir well.

Place cookie dough that has been shaped into a thick square on a piece of plastic wrap and chill a minimum of 6 hours or preferably overnight, and up to 3 days.

When ready to bake preheat oven to 400 degrees. Line baking sheets with parchment or a Silpat pad.

Scoop dough into approximately 1 T balls to make about 36 cookies. Do not press down.

Bake for 10-12 minutes or until the bottom of the cookie begins to turn golden brown. The top of the cookie may not look fully baked but it will still cook for a few minutes after removed from the oven.

Remove from cookie sheet and cool on wire rack.