In a microwave- safe bowl, combine the coarsely chopped chocolate, 1/4 cup water, and the butter. Microwave in bursts of 30 seconds stirring in between each burst for 20 seconds until the mixture is totally smooth. Allow to cool for 10-15 minutes.
Meanwhile, in a small heavy saucepan, whisk egg yolks, sugar, salt, and remaining 1/4 cup water. Cook and stir over medium heat until mixture reaches 160 degrees F or is thick enough to coat the back of a metal spoon. Remove from the heat and whisk in the melted chocolate mixture.
Set saucepan in a bowl of ice and whisk until cooled, about 5-10 minutes.
Meanwhile, whip the heavy whipping cream. Add it to a stand mixer along with the powdered sugar and vanilla. Whip until soft peaks form. Remove about 1/2 cup (to top mousse cups with). Fold the remaining in with the chocolate mixture. Mix until combined.
Spoon into the dessert dishes and refrigerate for 4 hours to overnight before serving. Serve with the reserved whipped cream and if desired, chocolate curls. To make the curls, take a room temperature chocolate bar and a vegetable peeler and peel the side of the chocolate bar over the mousse cups.