Preheat your oven to 180°C (350°F) and line an 8x8-inch metal baking pan with parchment paper.
In a large mixing bowl, combine the granulated sugar, vegetable oil, eggs, egg yolk, salt, espresso powder, and vanilla extract. Sift in the cocoa powder. Whisk vigorously until smooth and combined.
Melt the dark chocolate (in the microwave or over a double boiler) and whisk it thoroughly into the wet mixture until glossy.
Fold & Bake
Add the all-purpose flour to the bowl. Fold gently with a rubber spatula until just combined and no flour streaks remain. Do not overmix.
Pour the batter into the prepared baking pan and smooth the top with a spatula. Bake at 180°C (350°F) for about 30 minutes, or until the top is crinkly and set.
Cool & Slice
Remove from the oven and allow the brownies to cool completely in the pan for about 1 hour before slicing, so they set properly and don't crumble.
Notes
Room-Temperature Eggs: Ensure your eggs are at room temperature so they incorporate smoothly into the batter without seizing the warm melted chocolate.
Don't Overmix: Fold the flour in gently just until combined. Overmixing develops gluten and turns fudgy brownies into dry, cakey bars.
Weigh Your Flour: Measure your flour in grams using a digital kitchen scale to prevent packing excess flour into measuring cups.
Cool Before Slicing: Let the brownies cool completely in the pan for a full hour so they firm up and slice into clean, perfect squares.
Use a Metal Pan: Bake in a metal pan rather than glass or ceramic to ensure even heat conduction, crisp edges, and a fudgy center.