Line an 8x8 inch (20x20 cm) square pan with foil or parchment paper and set aside. Position a rack in the center of the oven and preheat to 325ºF (162ºC).

Over a double-boiler or in the microwave (30 second intervals), melt the butter, oil and sugar together. Then immediately whisk in the cocoa powder and espresso powder until combined. It will look a bit gritty, but don't fret, it will come together after we add the eggs. 

Whisk in your 2 eggs and 1 yolk until smooth.

Sift and fold in the flour, cornstarch and salt just until combined. Do not over mix. Finally mix in your chocolate chunks or chips.

Transfer batter to your prepared pan and top with more chocolate chunks on top if you'd like. Bake for 26 to 30 minutes. Do not overcook. Let it cool completely before cutting and serving. Enjoy!