Prep: Preheat oven to 350°F (175°C). Line a 9x9 inch metal baking pan with parchment paper, leaving an overhang on two sides. Lightly grease the paper.

Dry Ingredients: In a medium bowl, sift together the flour, baking powder, salt, cocoa powder, and espresso powder (if using). Set aside.

Butter Mixture: In a medium saucepan over medium heat, combine the butter, oil, and 1/3 cup (65g) of the sugar. Heat, stirring, until the butter is just melted. Remove from heat.

Egg Mixture: In a large bowl, vigorously whisk the eggs, egg yolk, vanilla, and the remaining 1 cup (200g) of sugar for about 30 seconds until pale and well combined.

Temper: Very slowly, in a thin stream, pour the warm butter mixture into the egg mixture, whisking constantly until fully combined and smooth.

Combine: Add the dry ingredients and chocolate chips to the wet ingredients. Using a rubber spatula, gently fold until just combined; stop when you no longer see dry flour streaks. Do not overmix.

Bake: Pour the batter into the prepared pan and smooth the top. Bake for 28-30 minutes, until the top is shiny and cracked, the edges are set, and the center has a slight jiggle.

Cool: Place the pan on a wire rack and let the brownies cool completely in the pan (at least 2 hours) before lifting out with the parchment paper and slicing.