Preheat the oven to 350°F (177°C). Grease the cups of a mini muffin pan with a generous amount of baking spray. Set aside.

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

While the chocolate-butter 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 2-3 minutes. Then, add the cocoa powder and stir until the batter is smooth.

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 slightly lightened in color and texture, about 3-4 minutes.

Fold the flour, instant espresso powder, and salt into the chocolate mixture just until the batter is fully combined.

Using a #60 cookie scoop, divide the batter between the mini muffin cups, until they're about 3/4 full. Bake for 10-12 minutes, until an inserted toothpick has just a few moist crumbs.

Let the brownies cool in the mini muffin pan until they're cooled and completely set, about 15-30 minutes. Use a small butter knife to gently release the brownies from the muffin cups. Sprinkle with flaky salt and enjoy!