Preheat oven to 325°F. Line a 9 × 9-inch light-colored metal baking pan with parchment paper, leaving some overhang to help lift the brownies out after cooling.

Place the butter in a microwave-safe bowl and microwave in short intervals until melted. Add the vegetable oil and stir to combine. If necessary, microwave for another 10–15 seconds until the mixture is warm but not sizzling.

Whisk in the Dutch-process cocoa powder and espresso powder until smooth. Set aside to cool slightly.

In the bowl of a stand mixer fitted with the paddle attachment, combine the granulated sugar, brown sugar, and room-temperature eggs.

Beat on medium-high speed for 3–4 minutes, until thickened, lighter in color, and well combined. Add the vanilla extract and mix briefly.

With the mixer running on low speed, slowly add the warm cocoa-butter-oil mixture. Mix until smooth and glossy, scraping down the sides of the bowl as needed.

In a separate bowl, combine the flour, cornstarch, and salt. Stir until evenly mixed.

With the stand mixer running on low speed, gradually add the dry ingredients, about one spoonful at a time. Mix just until incorporated, avoiding overmixing.

Add the semisweet chocolate chips and mix briefly on low speed until evenly distributed.

Spread the batter evenly in the prepared 9 × 9-inch pan.

Bake at 325°F, beginning to check at 25 minutes. The brownies are ready when the edges are set and a toothpick inserted near the center comes out with moist, fudgy crumbs rather than liquid batter.

Cool in the pan for at least 2 hours before cutting. For especially clean squares, refrigerate for another 30–60 minutes after cooling.

Lift the brownies from the pan using the parchment overhang and cut into 16 squares.