Preheat oven to 350 degrees (F) (175 degrees C). Line a 9×9-inch baking pan with parchment paper. Spray lightly with non-stick baking spray and set aside.

In a large bowl, sift together the flour, baking powder, salt, cocoa powder, and espresso powder. Set aside until needed.

In a medium saucepan, combine the butter, oil, and 1/3 cup of the sugar. Heat over medium heat, stirring frequently, until butter is completely melted. Remove from heat.

In a large mixing bowl, combine the eggs, egg yolk, vanilla (if using) and remaining sugar. Whisk until well combined, about 30 seconds.

Slowly, pour the warm butter mixture into the egg mixture, adding it very gradually (a little bit at a time) and whisking constantly until completely combined.

Add in the dry ingredients and chocolate chips and, using a rubber spatula, slowly stir until just combined. Do not over mix! Stop stirring when you see the last trace of dry ingredients.

Scrape the batter into the prepared pan and smooth the top. Bake for 28 to 30 minutes or until the edges are firm and the top is shiny and slightly cracked.

Place pan on a cooling rack and cool completely before slicing.