Toss the chocolate and espresso powder in a medium heat-proof bowl and set aside.
In the top of a double boiler over simmering water, combine the egg yolks and ¾ cup (6 oz/170 g) sugar and whiskey and stir constantly for 2-3 minutes, until the mixture thickens enough to coat the back of a spoon.
Immediately stir in the butter and water until melted and warmed through, then pour the mixture over the chocolate. Let this sit for 5 minutes without stirring to allow the chocolate to completely melt.
Whisk until combined, then strain into a medium bowl. Set aside.
In the bowl of a stand mixer fitted with a whisk attachment (or with a clean bowl and a handheld electric mixer), whisk the egg whites on high to soft peaks, about 2-3 minutes.
Add the remaining tablespoon of sugar and continue beating until the egg whites are stiff and glossy.
Fold ⅓ of the egg whites into the chocolate mixture to loosen the custard, then gently fold in the remaining egg whites until fully combined.
Divide the mousse among six small (4 fl oz/120 ml) serving glasses and refrigerate for at least 4 hours (and up to 1 days) to set.
To Make the Whiskey Cream
In the bowl of a stand mixer fitted with a whisk attachment (or with a clean bowl and a handheld electric mixer), combine the heavy cream, sugar and whiskey and whip on medium-high speed to soft, billowy peaks.
Dollop the whipped cream on top of the mousse and garnish with the chocolate shavings. Serve immediately.