In a microwavable bowl, microwave the butter and chocolate chips in 15 second intervals until fully melted and smooth. Set aside. In a mixing bowl, whisk the egg, egg yolk, sugar, vanilla, espresso powder, and salt. Add the chocolate mixture, whisking vigorously after adding to avoid the warm chocolate cooking the egg. Add the flour and cocoa powder, and divide into the two ramekins.
1/4 cup butter, 1/3 cup chocolate chips, 1 egg, 1 egg yolk, 1/4 cup sugar, 1 teaspoon vanilla, 1/2 teaspoon espresso powder, 1/4 teaspoon salt, 1 tablespoon flour, 1 tablespoon cocoa powder