In a medium saucepan, melt the chocolate with the butter over moderately low heat, stirring occasionally. Remove from the heat and stir in the vanilla.
In a medium, heatproof bowl, whisk the egg yolks with 1/4 cup plus 2 tablespoons of the granulated sugar and the coffee. Set the bowl over a saucepan of simmering water and whisk constantly until the yolks are frothy and an instant-read thermometer inserted in them registers 160°, about 6 minutes. Remove from the heat and fold in the melted chocolate.
In another medium bowl, whisk the egg whites with the cream of tartar and the remaining 1/2 cup of granulated sugar. Set the bowl over the saucepan of simmering water and whisk the whites constantly until an instant-read thermometer inserted in them registers 140°, about 5 minutes. Remove the bowl from the saucepan and continue beating the whites until stiff and glossy. Fold the beaten whites into the chocolate mixture.
In a clean bowl, beat 1 cup of the heavy cream until firm. Fold the whipped cream into the chocolate mixture until no streaks remain. Spoon the mousse into 8 glasses and chill for at least 2 hours.
Just before serving, beat the remaining 1 1/2 cups of heavy cream with the confectioners' sugar until firm. Sprinkle on the instant espresso and fold gently so the flecks and streaks are visible. Dollop the cream over the mousse and serve.
MAKE AHEAD The mousse can be prepared through Step 4 and refrigerated for up to 2 days.