Cake
Preheat the oven to 350º. Line 8 custard cups with aluminum foil.
Put the chocolate in a saucepan. Pour the hot water over the chocolate. Place over medium-low heat; add the sugar and stir with a wire whisk to dissolve the sugar and any unmelted chocolate. Add the butter gradually, a dollop at a time, whisking until the butter is incorporated before adding the next dollop. This should take about 10 minutes. Remove from the heat.
Slowly whisk the beaten eggs into the chocolate mixture. Add vanilla. Pour through a strainer into the foil-lined bowls. Discard any bits of egg that remain in the strainer. Bake 20 - 25 minutes until the batter rises and a cracked top crust forms. The mixture will still juggle, like molded gelatin. Resist the urge to bake it a little more, the butter and chocolate set up when chilled.
Let the cakes cool. Fold the foil over the tops and refrigerate at least 8 hours, keeping the cakes in the bowls.
Frosting
Gently remove the cakes from the foil. The cakes will have fallen in the center; if desired (I didn't) trim the cake to make a flat even surface. Invert on to a serving plate or small piece of parchment.
In a large mixing bowl, whip the cream with the sugar and the vanilla until the cream comes to soft peaks that hold their shape. Put the cream into a pastry bag fitted with a large star tip. Pipe stars, covering the dome completely. Garnish with white chocolate snowflake.
Snowflake Garnish
Melt white chocolate in a microwave safe bowl on 50% power for a few seconds at a time, stirring frequently until chocolate is melted. Put melted chocolate into a piping bag with a#1 or #2 tip.
On a flat baking tray, put printed template beneath a sheet of parchment paper and pipe melted chocolate on to the parchment in the snowflake shapes, moving the template as needed. Sprinkle with shimmering sanding sugar if desired.
Allow snowflakes to set up at room temperature, then chill in refrigerator until needed.