In the top of a double boiler over barely simmering hot water, melt the bittersweet chocolate and butter together, stirring occasionally until completely smooth. Remove from the heat and stir in the cocoa powder until no lumps remain. Set aside.

In a medium saucepan, combine the heavy cream, light corn syrup, sugar, and sea salt. Bring to a boil over medium heat, watching carefully. It comes up faster than you expect. Reduce to medium-low heat if needed and stir until the sugar dissolves completely, about 3 to 5 minutes. Remove from the heat immediately and stir in the vanilla extract and the chocolate mixture, scraping every bit in with a spatula.

Transfer to a bowl and press a piece of plastic wrap directly onto the surface to prevent a skin from forming. Let cool for 20 minutes before serving or jarring. Store in an airtight container or mason jar in the refrigerator for up to three weeks. To reheat, warm gently in a small saucepan over low heat, stirring until smooth, or microwave in 20-second intervals. The sauce is good at room temp too, slightly thicker and fudgier than when warm.