Preheat oven to 350F. Butter the bottom and sides of a 9×13 inch metal baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend up two sides of the pan and overhang slightly on both ends. Place another piece of parchment paper in the same manner, perpendicular to the first sheet. (This will make it easy to remove the bars from the pan after they have baked.) Butter the parchment.
In a large bowl, whisk cocoa, espresso powder, and boiling water together in large bowl until smooth. Add unsweetened chocolate and whisk until chocolate is melted. Whisk in melted butter and oil. Add eggs, yolks and vanilla and continue to whisk until smooth and homogeneous. Whisk in sugar until fully incorporated. Add flour and salt and mix with rubber spatula until combined.
Transfer 1/2 of the batter to prepared pan; using spatula, spread batter into corners of pan and smooth surface. Sprinkle with 3/4 of the chopped Snickers. Spread the remaining batter into the pan. Sprinkle with remaining Snickers. Bake 30 to 35 minutes, or until toothpick inserted into the center comes out with just a few moist crumbs attached. Transfer pan to wire rack and cool 1½ hours.
Using parchment overhang, lift brownies from pan. Return brownies to wire rack and let cool completely, about 1 hour. Cut into 2-inch squares and serve.