Preheat the oven to 325 degrees and line the bottom and sides of an 8x8 pan with parchment paper.
Melt the semi-sweet chocolate and the olive oil in a small saucepan over medium low heat, stirring constantly until creamy. Set aside.
Whisk the cocoa powder, flour, and salt in a medium bowl.
Beat the eggs, brown sugar, white sugar, and vanilla until combined.
Add the dry ingredients to the wet ingredients, including the melted chocolate, and mix with a rubber spatula until just combined. Do not overmix.
Fold in the chopped chocolate bar until combined. Do not overmix.
Line the bottom of the prepared pan with 16 Oreos.
Pour half the batter into the pan and spread it evenly.
Add 16 Reese's Cups in a single layer to the batter, pressing them down until just the tops show.
Pour the remaining batter into the pan and spread it evenly. Then, press more chopped Oreos into the top.
Bake the brownies for 33 to 40 minutes or until a toothpick comes out mostly clean with just a few moist crumbs. At this point, the top will begin to crack.
Cool the brownies in the pan for an hour. Then, transfer them to a wire cooling rack and cool completely.
Slice the cooled brownies and enjoy.
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Notes
I highly recommend using butter olive oil for these brownies to give them an extra-rich flavor. However, you can use regular olive oil if you don't have it.
Be sure to melt the chocolate and olive oil over low heat and stir constantly to prevent scorching.
Leave an overhang on either side or your pan when you line it. Those flaps can be used as handles to easily remove the brownies later before slicing.
Let your brownies fully cool before slicing them. This ensures they're nice and firm, giving you clean edges.
Use a sharp knife to cut the brownies and use minimal pressure. Letting the sharp knife do the work will prevent crushing.
Store these brownies in an airtight container for up to a week at room temperature.
To keep your brownies from drying out, add a piece of white bread to the container. The moisture from the bread will keep the brownies moist and soft.
NEVER refrigerate your brownies unless they're made with cream cheese or frosted with cream cheese frosting. The cold air of the refrigerator dries them out.