Time: 35 minutes, plus at least 4 1/2 hours to chill. Panna cotta means "cooked cream," and true to its name, our classic vanilla version of this recipe weighed in with nearly 20 grams of fat per serving. This svelte chocolate variation has a grown-up tang and tastes indulgent--yet has only 4 grams of fat.
In a small bowl, sprinkle gelatin over 1/2 cup milk. Let stand 1 minute, then stir and let stand about 10 more minutes for gelatin to soften.
In a medium saucepan, whisk remaining 1 1/2 cups milk, the sugar, semisweet chocolate, and cocoa over medium-high heat until steaming, about 3 minutes. Add gelatin mixture and whisk gently until it dissolves, 2 to 3 minutes. Let cool 15 minutes.
In a large bowl, whisk yogurt with vanilla until smooth. Pour and stir chocolate mixture through a fine strainer into yogurt, then whisk mixtures together.
Oil 8 ramekins (2/3 cup size). Divide yogurt mixture among ramekins. Chill, covered, until set, 4 1/2 hours and up to 2 days.
Run a small metal spatula between panna cottas and sides of ramekins to loosen. Immerse ramekins 1 at a time to just below rim in hot water until edges of dessert soften, 20 to 30 seconds; lift out and dry bottom of ramekin. Invert a small plate over each. Hold plate and ramekin together and give a firm shake to release panna cotta, easing it out gently with the spatula if needed.
Scatter chocolate shavings over desserts if you like.