Vanilla ice cream, chocolate or caramel sauce, halved strawberries, for serving
Preheat the oven to 450°. Butter and flour six 6-ounce ramekins. Place on a rimmed baking sheet and set aside.
In a medium, microwave-safe bowl, combine the butter and chocolate. Microwave at 50% power until the mixture is smooth and melted, about 2 minutes, stirring every 30 seconds. Set aside.
In the bowl of a stand mixer fitted with a whisk attachment, combine the eggs, egg yolks, sugar, vanilla, instant espresso granules, and salt. Beat on medium-high until thick and pale, about 3 minutes.
Immediately fold the warm chocolate mixture and flour into the whipped egg mixture. Divide the batter evenly among the prepared ramekins. (A heaping 1/3 cup should go in each).
Bake until the sides of the cakes are set, but the centers are still slightly jiggly, 8 to 10 minutes. Let the cakes cool in the ramekins for 1 minute. Invert each cake onto a serving plate. Serve immediately with ice cream, chocolate or caramel sauce and halved strawberries.