Preheat your oven to 425°F (220°C). This high temperature is key to achieving that perfectly molten center.
Grease your muffin tin with butter or non-stick cooking spray to ensure the cakes slide out easily after baking.
In a microwave-safe bowl, combine the dark chocolate and unsalted butter. Heat in the microwave in 30-second intervals, stirring in between, until fully melted and smooth.
In a separate mixing bowl, whisk together the confectioners' sugar, large egg, and egg yolk until light and fluffy.
Pour the melted chocolate mixture into the egg mixture and stir gently until combined. Then, sift in the all-purpose flour and optional espresso granules, folding until just combined.
Divide the batter evenly among the prepared muffin cups, filling each about 2/3 full.
Place the muffin tin in the preheated oven and bake for 12-14 minutes. The edges should be set, but the centers will still be soft.
Once baked, remove the tin from the oven and let it cool for 1 minute to help the cakes firm up slightly.
Carefully run a knife around the edges of each cake and invert the tin onto a serving plate. Serve warm, topped with a scoop of ice cream and a drizzle of hot fudge.