Preheat your oven to 350°F. Grease the bottom and sides of a 9 × 13-inch metal baking pan. Line the pan with parchment paper, leaving extra paper hanging over the edges to make lifting the brownies out easier. Add another strip of parchment in the opposite direction, then butter the parchment.
In a large bowl, whisk together the Dutch-process cocoa, instant espresso, and boiling water until smooth. Add the finely chopped unsweetened chocolate and whisk until completely melted. Stir in the melted butter and vegetable oil until combined.
Whisk in the eggs, egg yolks, vanilla extract, and sugar until the mixture becomes smooth and glossy. Add the flour and salt, mixing gently until just incorporated. Avoid overmixing to help keep the brownies tender and chewy.
Pour half of the brownie batter into the prepared pan and spread it evenly into the corners. Sprinkle half of the chopped Snickers bars over the batter. Spread the remaining batter on top, then scatter the rest of the Snickers pieces across the surface.
Bake for 30–35 minutes, or until the edges are set and the center no longer has a wet appearance. A toothpick inserted into the center should come out with a few moist crumbs attached.
Remove the pan from the oven and place it on a wire rack. Let the brownies cool completely, about one hour, before lifting them out using the parchment paper. Cut into squares and serve.
