How to Make It
Preheat oven to 300°F. Place chopped chocolates in a large heatproof bowl; set aside. Combine cream, milk, and tonka beans in a small saucepan. Bring to a simmer over medium-high, stirring often. Remove from heat. Set bowl with chocolate on top of saucepan. Let tonka beans steep 10 minutes. Pour cream mixture through a fine wire-mesh strainer into bowl with chocolates; discard tonka beans. Gently whisk chocolate mixture until completely smooth and incorporated, about 3 minutes.
Combine egg yolks, 3 tablespoons sugar, and kosher salt in a large metal bowl. Whisk until combined and pale, 1 minute and 30 seconds to 2 minutes. Gradually add chocolate mixture to egg mixture, whisking constantly, until fully combined. Pour mixture through a fine wire-mesh strainer into a large measuring cup or spouted bowl; discard solids. Divide mixture evenly among 4 (6-ounce) pot de crème jars or espresso cups. Place jars in a 4- to 6-inch deep roasting pan. Pour hot water into pan, letting water come up to 1 inch from tops of jars. Cover pan with aluminum foil. Fold up 2 corners to create openings for steam to escape.
Bake in preheated oven until edges of pots de crème are set and centers are slightly jiggly, about 50 minutes. Remove jars from water bath and transfer to a wire rack to cool completely, about 1 hour and 30 minutes. Chill until cold, at least 4 hours.
Combine gelatin and 1 tablespoon cold water in a small bowl; let stand 5 minutes. Whisk together coffee and remaining 1 teaspoon sugar in a separate bowl. Stir coffee mixture into gelatin mixture until dissolved. Spoon about 1 1/2 tablespoons onto each chilled pot de crème. Chill until coffee mixture is set, at least 2 hours. Sprinkle with sea salt, if desired, just before serving.