Combine the chocolate, water, cocoa powder, brandy and espresso powder in a medium bowl. Place the bowl over a small saucepan filled with about 1 inch of water set over low heat, so that it is barely simmering. Stir the mixture frequently until it is completely melted and smooth; remove the bowl from the heat.
In a large bowl, whisk together the egg yolks, 1½ teaspoons of the sugar and the salt until it is slightly thickened and pale in color, about 30 seconds. Pour the melted chocolate mixture into the egg mixture and whisk to combine. Let cool until it's slightly warmer than room temperature, about 5 minutes.
Whip the egg whites using a stand mixer with a whisk attachment on medium speed until foamy, about 1 minute. Add the remaining 1½ teaspoons of sugar and increase the speed to medium-high. Continue to whip until soft peaks form, about 1 more minute. Gently whisk one-quarter of the whipped egg whites into the chocolate mixture. Add the remaining egg whites and gently fold into the chocolate mixture using a rubber spatula until just a few white streaks remain.
Wipe out the mixing bowl and then add the heavy cream to the bowl. Again using the whisk attachment, whip the cream on medium speed until it begins to thicken, about 30 seconds. Increase the speed to high and whip until soft peaks form, at least another 15 seconds. Using a rubber spatula, gently fold the whipped cream into the mousse until no white streaks remain. Spoon the mousse into individual serving cups, cover with plastic wrap and refrigerate until set and firm, at least 2 hours or up to 24 hours.
If you are concerned about using raw eggs in this recipe, look for pasteurized eggs - almost all grocery stores carry them now.
You can omit the brandy if you do not drink alcohol. You may substitute vanilla extract, adding it to the egg yolks, sugar and salt in step #2.