unsweetened cocoa powder – for dusting (for serving)
Instructions
Line a 9×5-inch loaf pan with plastic wrap, leaving overhang on all sides. Chill the pan.
Melt the dark chocolate in a heatproof bowl set over barely simmering water, stirring until smooth, 3–5 minutes. Remove from heat and let cool until just warm to the touch.
Whip the heavy cream to soft peaks with a whisk or mixer, 2–4 minutes. Refrigerate the whipped cream.
Combine the granulated sugar and water in a small saucepan over medium heat. Stir just until the sugar dissolves, then stop stirring and simmer until the syrup reaches 240°F/115°C (soft-ball stage), 3–5 minutes.
While the syrup cooks, beat the egg yolks with the fine sea salt and vanilla extract in a stand mixer on medium-high until slightly thickened, 1–2 minutes. With the mixer running, slowly stream the hot syrup down the side of the bowl into the yolks. Continue beating until very thick, pale, and cooled to room temperature, 4–6 minutes.
Fold the melted chocolate into the yolk mixture with a spatula until smooth and uniform.
Gently fold the whipped cream into the chocolate base in two additions until no streaks remain.
Scrape the mixture into the prepared pan, smooth the top, and cover with the overhanging plastic wrap. Freeze until firm, 6–8 hours or overnight.
To serve, unwrap, invert onto a chilled platter, and peel away the plastic. Warm a knife under hot water, wipe dry, and slice. Dust lightly with unsweetened cocoa powder, if using, and serve immediately.