Preheat the oven and spray the pan: Preheat your oven to 350°F. Then spray an 8×8 inch square baking pan with cooking spray and line it with parchment paper.
Combine the dry ingredients: Sift together the flour, cocoa powder, salt, baking soda, and espresso powder into a large bowl and whisk together well. Then add one cup of the chocolate chips and mix it all together.
Dissolve the sugar in the butter and oil: Add the butter and oil to a small saucepan and melt it over medium heat. Next, add the sugar and whisk until the mixture is smooth and all the sugar crystals have dissolved. Now, remove the saucepan from the heat and allow the mixture to slightly cool before moving on to the next step in which the mixture needs to be hot but not scalding.
Temper the eggs: In a separate large bowl, whisk together the eggs, egg yolk, and vanilla extract. Now, very slowly pour the melted butter and sugar mixture into the whisked eggs until combined well, making sure to continue whisking the entire time.
Add the dry ingredients: Add the dry ingredients and slowly stir using a rubber spatula. The best method is to fold the dry ingredients into the wet ingredients until you no longer see any bits or chunks of flour. You want a smooth batter, but not over-mixed.
Prepare to bake: Pour the brownie batter into the prepared baking pan and smooth it out so that it is one even layer. Then sprinkle the top with the remaining chocolate chips.
Bake the best brownies: Put the brownies into the oven and bake at 350°F for 20 to 25 minutes. When done, the brownies will feel set on the edges, but the center should look very moist, yet not raw.
Cool and serve: Allow the brownies to cool in the pan for at least 10 minutes. They need this time to firm up a bit more and set. Then cut them into squares and devour them as is or serve with your favorite ice cream topped with a dollop of whipped cream!