Homemade caramel sauce is sandwiched between a layer of fudgy brownies and a layer of chocolate chip cookie dough in these from-scratch brookie bars.
Using an electric mixer, beat together the butter and both sugars until light and fluffy. Reduce speed to low and add the egg, then the egg yolk, then the vanilla, mixing to incorporate after each addition.
Add the flour, baking soda, and salt, then mix until combined. Using a rubber spatula or a wooden spoon, stir in the chocolate chips. Cover and chill*.
Over medium heat, melt the sugar in a small saucepan. Stir with a wooden spoon until the granules have melted completely (they will begin to clump together, then melt into an amber-colored liquid). It's important that the sugar is completely liquified, so be patient!
Reduce heat to low and add the cubed butter (the sugar may bubble and splatter when the butter is added, this is okay). Continuing stirring until the butter is melted and completely incorporated with the melted sugar.
While continuing to stir the mixture, slowly pour in the heavy cream (the mixture may bubble and splatter again). Mix until incorporated, then increase heat to medium again. Allow caramel to boil for another minute or so, stirring occasionally.
Remove the caramel from the heat and stir in the vanilla extract and salt. Set aside to cool slightly.
Preheat oven to 350 degrees Fahrenheit. Line a 9x13 inch baking pan with parchment paper and spray with cooking spray.
In a large, microwave safe bowl, combine the butter and chocolate. Microwave at 30 second intervals until melted completely, stirring between each interval.
Whisk in both sugars. Add the eggs one at a time, whisking after each addition. Then, whisk in the vanilla extract.
Using a rubber spatula, fold in the flour, cocoa powder, and salt until combined.
Spread batter evenly in the prepared baking pan. Bake at 350 degrees for 18 minutes (the top should be just barely starting to set, but still shiny). Meanwhile, make sure the caramel sauce is a good consistency for drizzling. If it's cooled too much, you can warm it up briefly in the microwave (10 second intervals).
Remove the brownies from the oven & drizzle caramel sauce all over the top. If needed, use the back of a spoon to spread the caramel gently.
Drop tablespoons of cookie dough on top of the caramel layer (I use my fingers to flatten the tablespoons of dough slightly into discs, but no need for perfection here) until the brownies/caramel are mostly covered.
Return the baking pan to the oven and bake for another 20-24 minutes, until the cookie layer is golden brown on top. It should be mostly set, but still slightly jiggly for ooey gooey brookie bars!
*Cookie dough can be prepared up to 2 days in advance and kept covered in the refrigerator. Let it sit at room temp while preparing the brownies so it's easier to work with.