1. Preheat the oven to 350°F.
2. Grease a 9x13-inch pan and line the bottom and sides with parchment paper.
3. Allow the butter to soften for at least 20 minutes.

For the Cookie Dough:

1. Combine the softened butter with the brown and granulated sugar. Beat on medium speed until well combined, 2-3 minutes.
2. Next add the egg, egg yolk and vanilla extract and continue beating until well incorporated.
3. In a separate bowl, combine the flour, salt and baking soda.
4. Stir to combine and slowly add the dry ingredients into the dough, mixing on low speed until dough starts to form.
5. Before the dough thickens, add the chocolate chips. Beat until incorporated. Set aside.

For the Brownie Layer:

1. Chop the chocolate into smaller pieces and melt the chocolate in a microwave-safe bowl for approximately 2 minutes at 50% power.
2. Stir occasionally and microwave until completed melted and smooth. Set aside to cool.
3. Melt the butter in a microwave-safe bowl and combine with the granulated sugar; beat together until well combined.
4. Next add the egg and egg yolk and beat for 60 seconds at a higher speed until the batter is fluffy (texture resembling a cake batter).
5. Then add the vanilla extract and melted chocolate and beat until combined.
6. Combine the dry ingredients in a separate bowl and stir to combine.
7. Add the dry ingredients to the batter and mix just until combined.

Assembly:

1. Pour the prepared brownie batter in the bottom of the pan.
2. Then take large pieces of the cookie dough and press it flat with your hands, then press it into the brownie batter.
3. Continue to layer all the remaining cookie dough.
4. Bake at 350°F for 35-40 minutes. I suggest covering with aluminum foil after 15-20 minutes to prevent too much browning on top.
5. Test for doneness by inserting a toothpick into the center. The brownie should be a little fudgy but not wet.