Put the chocolate in a heatproof bowl. Bring the cream almost to a boil in a small saucepan over medium heat, then pour it immediate only the chocolate. Let stand for 4-5 minutes to melt the chocolate.
When the chocolate is melter, add the fine sea salt and whisk gently until smooth. Add the vanilla and whisk gently to incorporate completely. Cover loosely and refrigerate until firm, at least 2 hours.
Using a teaspoon or a melon baller, scoop about 1 teaspoon of the chocolate mixture and roll into a 3/4-inch round truffle. Roll the truffle in cocoa powder and gently press 1 crystal of coarse sea salt into the top of the truffle to secure it. Place on a plate or in a container, and continue until all the truffle mixture has been used.
Return the truffles to the refrigerate to chill until firm, at least 20 minutes. The truffles will keep in an airtight container in the refrigerate for up to 1 week. Serve chilled or at room temperature.