Combine espresso and butter in the top of a double boiler over simmering water.
Place dark chocolate pieces in the espresso mixture; cook without stirring until chocolate starts to melt, about 3 minutes. Whisk chocolate and espresso mixture until well combined. Remove from heat and set aside.
Combine Marsala wine, egg yolks and sugar in a saucepan over medium-low heat, whisking constantly until frothy and thickened, 3 to 5 minutes. Remove from heat.
Stir mascarpone cheese into Marsala mixture.
Combine chocolate mixture with mascarpone mixture. Cool to room temperature.
Whip cream in a bowl until soft peaks form, 2 to 3 minutes.
Fold half of whipped cream into cooled chocolate mixture.
Fold second half of whipped cream into chocolate mixture. Cover with cling film and chill for at least 2 hours.
Not a mousse. A very rich cream. I should've known from the lack of egg whites. The air incorporated during the whisking of the egg yolks & sugar is totally insufficient to give the end product a light texture. Great as a cake topping or filling as suggested but a misleading title. - 26 Sep 2016