Lightly grease a 6-8 ounce ramekin or small baking dish with butter or cooking spray—this ensures your brownie releases cleanly after cooking. While your air fryer preheats to 350°F, gather all your ingredients and measure them out. I like to use room temperature egg yolks because they emulsify more smoothly into the batter, creating a fudgier texture rather than a cake-like one.

Melt the butter in a small bowl (about 30-45 seconds in the microwave or gently over low heat), then whisk in the sugar, cocoa powder, vanilla extract, salt, and espresso powder. The espresso powder is a secret weapon—it deepens the chocolate flavor without adding coffee taste. Stir until smooth and well combined, making sure there are no cocoa lumps. This warm chocolate mixture is your flavor foundation, so take a moment to ensure it's fully integrated.

Add the room temperature egg yolk to the warm chocolate mixture and stir vigorously for about 30 seconds until the yolk is fully incorporated and the mixture becomes glossy and emulsified. This step is crucial—the warm chocolate gently cooks and binds with the yolk, creating richness and helping the brownie set properly in the air fryer. Don't skip the vigorous stirring; it ensures a cohesive batter rather than separated ingredients.

Gently fold in the flour until just combined—you want a thick, fudgy batter with no streaks of white flour, but don't overmix or you'll develop too much gluten and lose that tender crumb. Pour the batter into your prepared ramekin, smoothing the top lightly with the back of a spoon. The batter should fill the ramekin about two-thirds full to allow for some rise.

Place the ramekin in your preheated 350°F air fryer basket and cook for 10-12 minutes. The brownie is done when the edges are set and slightly pulling away from the sides, but the center still jiggles slightly when you gently shake the ramekin—this is what gives you that fudgy, almost molten center. If you prefer a cake-like brownie, air fry for the full 12 minutes or slightly longer until the center is fully set.

Remove the ramekin from the air fryer and let it cool for 3-5 minutes at room temperature. This resting period allows the brownie to set slightly while staying warm and fudgy. Serve it directly in the ramekin with a spoon, optionally topped with a scoop of vanilla ice cream, whipped cream, or a light dusting of cocoa powder if desired.