Cut the butter into smaller bits and throw them into a liquid measuring cup with the broken bits of chocolate, espresso powder, and salt. Microwave it on high for 30 – 45 seconds, until the chocolate is mostly melted but some chunks are still present. Whisk until the chocolate is melted and everything is smooth with a fork. Set aside for a minute or two.
Crack open the egg and dump the white into one bowl and slide the egg yolk into the chocolate cup and whisk quickly until absolutely homogenously combined.
With a clean whisk whip the egg whites until they get to medium peaks, sprinkle in the sugar and whisk until they form stiff peaks. In a separate bowl, whisk the heavy cream until it, too, forms into stiff peaks. Splash in the Campari and whisk again just to combine it all.
To the chocolate mixture, plunk in about a third of the whites and beat until it’s all combined, lightening the chocolaty, eggy base. Gently fold in the rest of the egg whites and the heavy cream into the base until completely combined, still taking care not to deflate the whites and cream as you do.
Decant into a glass and stash back in the freezer for at least 20 minutes to set or in the fridge for at least 2 hours. You can top it with whipped cream and shaved chocolate if you want or have the patience but if you don’t I wouldn’t worry too much about it.
Campari works so well with chocolate here but if you need something a little less bitter give it a splash of Cointreau instead. More recently I've made this with orange bitters (Angostora) because I ran out of Campari (prosecco and Campari will do that) and it sooo works. Don't feel limited to orange either; I've used amaretto, Frangelico, framboise, and, my most recent discovery, an ancho chili liqueur called Ancho Reyes. You won't regret that one.