To make a similar white chocolate disc to use as a garnish for each slice, just pour melted gluten free white chocolate into a lightly oiled, round chocolate/candy mold and refrigerate until set.
Egg-Free:
For egg-free, I suggest making the buttermilk cake unless you are using liquid from a can of cannellini beans as the egg substitute. Other egg substitutes don't turn out as well if the recipe contains a lot of eggs, as is the cake with sponge cake.
Cannellini Bean Egg Susbtitute Recipe:
Replace each egg with 2 tablespoons of cannellini bean liquid + 1 tablespoon fat such as butter, refined coconut oil, or hydrogentated palm oil.
https://glutenfreerecipebox.com/gluten-free-blackout-cake/