In a small saucepan over low to medium-low heat, melt butter and chopped dark chocolate together, stirring frequently until smooth. Remove from heat and cool for 5–8 minutes.
Preheat oven to 425°F (220°C). Generously spray four ramekins with baking spray (or butter and lightly dust with cocoa powder) and place on a baking sheet.
In a medium bowl, beat egg, egg yolks, and sugar using a hand mixer on medium speed for about 2 minutes until slightly thickened and pale.
Add flour, salt, and vanilla extract. Beat again for 1–2 minutes until smooth.
With mixer on low, slowly drizzle in melted chocolate mixture until just combined. Batter will be thick and glossy.
Divide batter evenly among prepared ramekins.
Press one piece of dark chocolate into the center of each ramekin.
Bake for 10–11 minutes until edges are set but centers remain soft.
Cool for 5–10 minutes. Run a knife around edges, place a plate on top, and carefully invert to release.
Serve warm with optional vanilla ice cream or whipped cream.