Preheat your oven to 425°F (218°C). Thoroughly grease four 6-ounce ramekins with butter, then dust with cocoa powder.
In a microwave-safe bowl, combine chopped chocolate and butter. Microwave in 30-second intervals, stirring between each until completely melted and smooth.
In a separate bowl, whisk together the flour, sugar, and salt (plus espresso powder if using).
Add eggs and egg yolks to the dry ingredients and whisk until completely combined.
Pour the melted chocolate mixture into the egg mixture and gently fold together until combined. Be careful not to overmix – 12-15 gentle folding motions should do it.
Divide the batter evenly among the prepared ramekins, filling each about ¾ full.
Place ramekins on a baking sheet and bake for 12-14 minutes until the edges are firm but the centers are still soft. The tops should be set but slightly jiggly in the middle.
Allow to cool for exactly one minute before running a knife around the edges and inverting onto serving plates.
Serve immediately with vanilla ice cream or fresh berries if desired.