Preheat the oven to 350 degrees Fahrenheit. Line a 9x9 baking pan with parchment paper and set aside.
To make the brownie layer: in a small bowl, add the butter and chocolate chips. Melt in the microwave in 30 second increments, stirring between each until fully melted.
In a second bowl, add the sugar, vanilla and egg. Use an electric mixer to beat for about 30 seconds. Add in the chocolate mixture and beat to combine. Fold in the cocoa, flour and salt. Mix until well combined. Transfer to the prepared baking pan and spread smoothly.
To make the cookie layer: Add the softened/room temperature butter, white sugar and brown sugar to a mixing bowl and beat with an electric mixer for about 30 seconds. Add in the egg yolk and vanilla and beat to combine.
Add in the flour (spoon and level to measure), baking powder, baking soda and salt. Mix to combine. Fold in the chocolate chips.
Divide the dough into about 4 large balls. Flatten each ball in your hand and place on top of the brownie layer, pressing into the corners of the pan. Take any leftover dough and press to fill in any gaps.
Top with extra chocolate chips if desired. Bake for 25-30 minutes or until the edges are just golden brown and the center looks set. Let cool fully in the pan, then slice and enjoy!