Preheat to 350°F. then line an 8x8-inch metal pan with parchment and set aside.
In a heatproof bowl, combine butter, chopped chocolate, and oil. Melt gently (microwave in 30 sec bursts or over a double boiler), then stir in cocoa powder and the espresso powder until smooth. Let it cool slightly.
½ cup unsalted butter, 3 tbsp neutral oil, 4 oz 70% dark chocolate, chopped, 2/3 cup Dutch process cocoa powder, 1 tsp instant espresso powder
In a large bowl, beat eggs, yolk, granulated sugar, powdered sugar, and vanilla on high speed for 3-6 minutes until thick, pale, and ribbon-y. This is where that shiny crackle top is born!
2 large eggs, 1 egg yolk, 1 cup granulated sugar, ½ cup powdered sugar, 1½ tsp vanilla extract
With the mixer on low, slowly pour in the slightly cooled chocolate-butter mixture.
Sift the dry ingredients into the brownie batter mixture. Gently fold into the batter with a spatula.
½ cup all-purpose flour, 1 tbsp cornstarch, ½ tsp kosher salt
Pour the batter into the prepared pan, smooth the top, and tap the pan once on the counter. Bake 25 minutes, or until a toothpick comes out with moist crumbs
Let cool in pan for a little bit before adding the toppings.