Line a 9x13 pan with wide heavy duty foil, leaving a couple inches over the edge (to pull brownies out after baking). Spray the foil with cooking spray.
Spray oil
Boil a pot of water. Meanwhile, whisk cocoa and espresso powder in a large mixing bowl. Measure boiling water, then pour into cocoa espresso mixture, and whisk until blended.
1 teaspoon espresso powder, 1/3 cup Dutch processed cocoa, ½ cup plus 2 tablespoon boiling water
Add dark chocolate into the mixture and whisk until melted, and chocolate is melted.
2 ounce semi sweet dark chocolate
To this chocolate mixture, add in melted butter and oil. Stir until mixed.
4 Tblsp butter, ½ cup vegetable oil + 2 tablespoons
Now it is time to add in the eggs, egg yolks and vanilla. Continue to stir until smooth. Add in sugar and mix until smooth.
2 large eggs, 2 egg yolks, 2 ½ cups sugar, 2 teaspoons vanilla extract
Whisk in flour, and salt until incorporated. Stir in milk chocolate chips.
1 ¾ cups all purpose flour, ½ teaspoon salt, 8 ounce high quality milk chocolate chips
Scrape batter into foil lined pan. Bake 35 minutes, turning pan half way thru. Test with toothpick, watching for a few moist crumbs attached.
Transfer pan to wire rack and let cool completely, about 1 hour. Remove from pan using foil liner to lift brownies out of pan. Cut when cooled.
Notes
Brownies keep best in an airtight container, between small squares of parchment paper, for about a week. That is, if they last that long!