Preheat the oven to 375°F (190°C). Grease an 8×8'' pan with butter or baking spray and line the pan with parchment paper, leaving an overhang.

In a large microwave-safe mixing bowl, combine the butter and bittersweet chocolate. Melt the butter and chocolate together in 30-second increments, stirring in-between rounds, until it's smooth and fully melted.

Next, whisk in the cocoa powder, oil, and espresso powder until you have a smooth, silky mixture. While the mixture is still hot, add the sugar. Using a hand mixer, beat the sugar into the butter-chocolate mixture until the sugar is mostly dissolved, about 3 minutes.

Then, add the eggs one at a time followed by the vanilla extract. Turn the mixer to high and beat the chocolate mixture until it's lightened in color and texture, about 3-4 minutes.

Fold the flour and salt into the chocolate mixture until there are just a few streaks of flour left. Then, add the mini chocolate chips and mix just until the batter is fully combined.

Spread the batter evenly in the prepared 8×8 pan. Bake the brownies for 27-32 minutes, until the edges are set and the center is still a bit underdone. To test if the brownies are done, insert a toothpick into the area in-between the edges and center of the brownies. If the toothpick has just a few moist crumbs, they're ready to go.

Let the brownies cool completely at room temperature. Then, cover and refrigerate the brownies for 1-2 hours (optional – for easy slicing). Using the edges of the parchment paper, remove the brownies from the pan and cut them into twelve 2×2-inch squares. Enjoy!