Fill the cavities of two half circles silicone molds
3/4 full, you will fill about 10 cavities (see notes if you have only one mold). Use the back of a spoon to slightly flatten the top. Transfer to the freezer and freeze until hard, at least 2 hours. You will have chocolate mousse batter left, just transfer it to ramequins and place in the refrigerator. Dessert bonus. In the meantime, prepare the vanilla cake.