In a double boiler over low heat,  or a medium glass or metal bowl placed over a saucepan filled with 1-inch of simmering water, add the chocolate, cocoa powder, espresso, and orange liqueur. 

Melt the chocolate, stirring constantly with a heatproof spatula, until melted and combined – about 2 minutes.  Remove from the heat and set aside to cool for about 8 minutes.

Meanwhile, in a medium bowl using a hand-held mixer on high speed, beat the egg whites until frothy – about 30 seconds.  Sprinkle in 1 tablespoon of sugar and continue beating until the whites are at the firm peak stage and are thick and glossy – about 1 minute longer.  Set aside.

In another medium bowl, whip ¾ cup of the heavy cream with the handheld mixer until it is doubled in volume and holds a soft peak – about 2 minutes.  You should have about 1 ½ cups of whipped cream.

In a large bowl using the hand mixer on high speed,  beat the egg yolks and 1 tablespoon of sugar until very thick and pale – about 3 minutes.

Add the vanilla and salt.  With the mixer on low speed, add the cooled chocolate mixture.  Mix just until combined.

Using the spatula, gently fold in ¼ of the beaten egg whites.  Fold until just combined, but some white streaks remain. Add the remaining egg whites and continue to gently fold.

Spoon in the whipped cream and continue folding to combine the ingredients.  Cover the bowl with plastic wrap and chill for at least 2 hours or up to 24 hours.

30 minutes before serving, using the handheld mixer at medium speed, beat in the last ½ cup of heavy cream until it begins to thicken.  Add the remaining 1 tablespoon of sugar. Continue to whip the cream until soft peaks form – about 1 minute longer. You should yield about 1 cup of whipped cream.

Spoon the mousse into serving dishes or small ramekins.  Garnish with a dollop of the whipped cream. Serve cold.