In the bowl of a stand mixer fitted with the whisk attachment, combine the egg yolks and icing sugar and whip on high speed until doubled in volume.
Place the chocolate pieces in a stainless-steel bowl and set the bowl over a pot with a few inches of water in it. Heat over low heat until the chocolate is melted.
Bring the cream to a simmer in a pot over medium heat. Remove from the heat and slowly add the cream to the chocolate, stirring with a rubber spatula, to form a ganache.
Add the ganache to the yolk mixture and whisk to combine, scraping down the sides of the bowl as needed. Add the butter and whisk to combine. With the mixer running, slowly stream in the hot water, and mix until very smooth and emulsified. Pour over the prepared fondant base, spread to an even layer, and return to the fridge. Chill for 1 hour, then cover with plastic wrap and freeze until firm.