Step 1: Preheat your oven to 425°F (218°C). Grease two 6-ounce ramekins with butter and dust them with cocoa powder or flour, tapping out the excess.
Step 2: In a microwave-safe bowl, combine the chopped dark chocolate and butter. Microwave in 30-second increments, stirring in between, until completely smooth.
Step 3: In a separate medium bowl, whisk the egg, egg yolk, powdered sugar, vanilla extract, and salt until the mixture is pale and slightly frothy.
Step 4: Gently fold the melted chocolate mixture into the egg mixture until well combined. Sift the flour over the batter and fold it in until just incorporated; do not overmix.
Step 5: Divide the batter evenly between the prepared ramekins. Place them on a baking sheet and bake for 12-14 minutes, or until the sides are firm but the center still jiggles slightly.
Step 6: Let the cakes cool in the ramekins for 1 minute. Carefully run a knife around the edges and invert them onto serving plates. Serve your midnight chocolate lava cake immediately while warm.