Preheat the oven to 450°F. Butter and coat 2 ramekins with cocoa powder. Set aside.

In a microwavable bowl or double boiler, melt the chocolate, salt, and butter. If using a microwave, stir every 30 seconds. Once melted, add vanilla and espresso powder. Set aside.

In a medium mixing bowl, whisk the egg, egg yolk, and sugar until pale yellow and slightly thick. The mixture should form a ribbon when the whisk is lifted.

Fold the melted chocolate mixture into the egg mixture. Then fold in the flour until evenly combined.

Divide the batter between the ramekins. Bake for about 10 minutes until the center is soft but the outsides are firm (the center should have a slight indent).

Cool for 2 minutes, then flip onto a plate. Let sit for 10 minutes before removing the ramekins. Serve immediately.