Preheat the oven to 350°F. Grease an 8x8 pan and line with parchment. Set aside.
Start by adding 2 sticks of unsalted butter to a heavy bottomed saucepan. Melt it over medium heat. If you want, continue heating it until the butter has browned. Here's our guide for how to brown butter for step-by-step instructions! The key is, no matter if you're browning the butter or not, the butter needs to be hot enough to melt the chocolate easily! After the butter is nice and hot (and potentially browned) remove the saucepan from the heat and add in 30g of cocoa powder, the chopped chocolate bar, and 2 teaspoons of instant espresso. Keep mixing everything together with a rubber spatula to melt the chocolate and combine everything until smooth. Once combined, remove from the saucepan and place in a separate container to cool.
In a large bowl, using an electric mixer, mix together 2 eggs, the egg yolk, 125g of white sugar, and 125g of light brown sugar until pale and fluffy. This will take 3-4 minutes.
Once the chocolate mixture has cooled down, stream it in slowly to the egg and sugar mixture, along with 1 tablespoon of vanilla extract while using the hand mixer on medium speed to incorporate it. Be sure to scrape the bowl a few times with a rubber spatula to make sure everything is incorporated.
Now, slowly sift in ¾ cup of all purpose flour and 1 teaspoon of kosher salt to the wet ingredients. Mix the batter on low speed with the electric mixer to incorporate gently. Once almost all the way mixed, switch to a rubber spatula to ensure you don't overmix the brownies at this stage.
When everything is almost all the way incorporated fold in ¾ cup of semi-sweet chocolate chips.
Pour half of the batter into the prepared baking pan.
Swirl in about half of the caramel (about ¼ cup) to create ribbons. Layer on about half of the chopped Oreos (6 Oreos) and peanut butter cups (3 large Reese's cups), then cover them with the rest of the batter.
Top the brownies with the rest of the salted caramel sauce, the remaining chocolate chips, chunks, Oreo pieces, and chopped Reese's cups.
Bake for roughly 45-50 minutes, testing for doneness by inserting a knife into the center of the brownies beginning at 35 minutes. You’ll know they’re done when the edge is set, but a toothpick inserted into the middle still comes out with a few moist crumbs attached to it.
As soon as the brownies come out of the oven, sprinkle them with sea salt. Allow them to cool completely in the pan and trasnfer to a cooling rack. This will take about an hour.
Cut into 9-12 pieces, and serve. Enjoy!
Store leftovers at room temperature in an airtight container for up to 5 days.