Steps
1. Preheat your oven to 425°F (220°C). Grease a ramekin with butter and dust it with cocoa powder.
2. In a microwave-safe bowl, combine the dark chocolate and unsalted butter. Microwave in 30-second intervals until melted and smooth, stirring in between.
3. Whisk in the powdered sugar until fully combined.
4. Add the whole egg and egg yolk, whisking until the mixture is smooth and glossy.
5. Gently fold in the flour and salt until just incorporated.
6. Pour the batter into the prepared ramekin, filling it about 3/4 full.
7. Bake in the preheated oven for 12-14 minutes, until the edges are firm but the center is still soft.
8. Remove from the oven and let it sit for 1 minute. Carefully invert onto a plate.
9. Serve warm, preferably with a scoop of vanilla ice cream.