In a bowl, mix together the flour, baking soda, salt, and cornstarch for the cookies. Set aside.
Beat the butter and both sugars together in a separate bowl until light and fluffy. Beat in the egg, egg yolk, and vanilla until well combined.
Slowly add the flour mixture, mixing in the flour as you add more. Once all the flour is added and there are no streaks of flour fold in the chocolate chips and mix just until they are evenly distributed.
Cover the cookie dough and chill for at least four hours, preferably overnight.
Once the cookie dough is made, immediately start the brownie batter.
Mix together the flour, cocoa powder, and salt for the brownies in a small bowl. Set aside.
In a medium saucepan, add the butter, sugar, and chocolate together. Heat on low and mix until the butter and chocolate are melted and the mixture is well combined. It will look fairly grainy at this point. Remove from heat and allow to cool until it is room temperature. If you kept the stove on low, it shouldn't take took long to reach this point.
Vigorously. beat the vanilla and egg into the chocolate mixture until well combined.
Stir the flour mixture into the chocolate mixture and mix until just combined and there are no streaks of flour.
Transfer the brownie batter into a bowl, cover, and chill for at least four hours, preferably overnight.
Remove the cookie dough and brownie batter from the refrigerator and uncover.
Place parchment paper or silicone baking mats onto baking sheets.
Measure out ½ tablespoons (1 ½ teaspoons) of the brownie batter. The batter should be thick, and may require a few minutes at room temperature or a touch of muscle to scoop it out. Roll out the brownie batter into a ball.
Measure out one tablespoon of cookie dough. Flatten slightly and create an indentation. Nestle the brownie batter ball into the indentation. Measure out another tablespoon of cookie dough and use that to cover the brownie ball. Roll the cookie dough in your hands swiftly to form a large ball. Set aside and repeat until you've run out of cookie dough. (You might end up with a bit of leftover brownie batter. I put mine in lined mini muffin tins to make brownie bites.)
Chill the cookie dough balls together as you preheat the oven to 350 degrees F.
Bake the cookies for about 10 - 12 minutes. Leave on the baking sheet for about 2 minutes to cool before placing on wire racks to cool completely.