Preheat oven to 325 degrees. Prepare a cookie sheet by lining with parchment paper, spraying with cooking spray, or use a silpat liner.
In a large mixing bowl or the bowl of a stand mixer, combine melted and cooled butter, brown sugar, and sugar. Blend until combined.
Add egg, egg yolk and vanilla. Blend until fluffy looking and light in color. About 2 minutes.
Add flour, baking soda, and salt. Blend just until combined. Fold chocolate chips into the dough and mix until combined.
Use a 1/3 cup measuring cup and divide dough into 10 large balls.
Place a dough ball in the palm of your hand and flatten it. Place a fun size Hershey bar (broken in half and place on top of each other) in the middle of the flattened dough. Bring the sides up and roll into a ball. Place on cookie sheet. Repeat with the remaining dough balls. Place 4 cookies per cookie sheet.
Bake 13-15 minutes or until browned around the edges while the middle still looks slightly undercooked. Let cool on baking sheet for 15 minutes to give them time to finish cooking and come together.
Eat a cookie warm for a gooey chocolate center. Once the cookies cool completely the chocolate does harden up in the middle. Eat as is or warm up the cookie in the microwave for a few seconds.