Preheat oven to 350°F (175°C). Thoroughly grease four 6-ounce ramekins with butter, then dust with cocoa powder.
Combine melted butter, melted chocolate, and sugar in a medium bowl and whisk until smooth and glossy. Add espresso powder if using.
Whisk in eggs one at a time until completely incorporated. Add vanilla extract if using and mix until just combined.
Gently fold in the sifted flour and salt using a rubber spatula until just incorporated. Don’t overmix.
Fill each prepared ramekin halfway with batter. Add 1 tablespoon of caramel sauce to the center of each, then top with remaining batter, leaving 1/4 inch space at the top.
Place ramekins on a baking sheet and bake for 12-14 minutes until edges are set but center still jiggles slightly.
Allow cakes to cool for exactly 1 minute after removing from oven.
Run a knife around the edges of each ramekin to loosen. Place a dessert plate on top and carefully invert. Lift the ramekin to release the cake.
Drizzle with additional warm caramel sauce and serve immediately with ice cream, berries, and a pinch of flaky sea salt if desired.
Notes
For best results, don’t overbake – the cakes should have a slightly jiggly center.
You can prepare the batter up to 24 hours ahead, fill ramekins, cover with plastic wrap, and refrigerate until ready to bake.
If baking from refrigerated, add 1-2 minutes to the baking time.
Nutrition facts are estimates and may vary based on ingredients used, portion sizes, and preparation methods.