Step 1

Coat 2 (8- to 10-ounce) ramekins with baking spray with flour. Fill a medium pot with water to a depth of 1 inch; bring to a simmer over low. Set a medium-size heatproof glass or metal bowl over pot. Add butter and 3 ounces (about 1/2 cup) of the chocolate to bowl. Cook, stirring occasionally, until melted and smooth, about 3 minutes. Remove from heat; let cool slightly, about 10 minutes.

Step 2

Whisk egg, granulated sugar, brown sugar, espresso granules, and vanilla into cooled butter-chocolate mixture until well combined, thick, and glossy. Add flour and kosher salt; stir until combined. Pour batter evenly into prepared ramekins (about 1/2 cup batter each).

Step 3

Place ramekins in basket of an air fryer. Bake at 300°F (do not preheat air fryer) until brownie tops and sides are set and a toothpick inserted into the center comes out with some moist clumps attached, 16 to 19 minutes. Remove brownies from air fryer. Sprinkle evenly with remaining chocolate and garnish with flaky sea salt. Let cool 15 minutes for gooier brownies, or 30 minutes for firmer brownies. Serve with vanilla ice cream, if desired.