Line a 9x13 pan with parchment paper or foil. If using foil spray with cooking spray or baker's joy. Adjust oven rack to lowest position and heat to 350.
In a large bowl, whisk the espresso powder, cocoa powder and boiling water until smooth. Add the chopped unsweetened chocolate and whisk until combined.
Whisk in the melted butter and oil. The mixture will look curdled at this point and that is ok.
Add the eggs, yolks, salt, and vanilla and whisk.
Whisk in the sugar and keep whisking until it is fully incorporated. It helps to whisk and then let the batter rest, and then whisk again to let the sugar dissolve a bit.
Add the flour and use a rubber spatula to gently fold in the flour. When you can still see some white streaks, add the chocolate chips or chunks and keep folding until it is combined but don't over mix.
Pour into the prepared pan and bake until a toothpick inserted half way between the center and the edge is clean or has a few cooked crumbs attached. Brownies are best when they are slightly under baked, so you do not want to over bake these. Depending on your oven this can be anywhere from 20-35 minutes so keep an eye on them.
Transfer to a rack to cool for 90 minutes. Remove from pan and let fully cool. Cut into squares.
If making the variation with both oreos and pb cups, slightly crush the oreos and mix half into the brownie batter before pouring into the pan. Spread half the batter in the pan, top with the pb cups and then spread the other half over the top. Sprinkle the other half of the oreos on top and bake.
If making just the oreo variation like these I made today, crush 16 of the oreos and keep 16 whole. Mix half the crushed in with the brownie batter and layer the other 16 in between layers of batter like instructed for the pb cups. Then top the brownies with the rest of the crushed oreos.