Combine the cooled brewed coffee, unsweetened cocoa powder, granulated sugar, and vanilla extract in a medium bowl. Whisk until smooth.

Separate the egg yolks from the egg whites. Add the yolks to the mocha mixture and whisk until fully incorporated.

In a large mixing bowl, beat the heavy cream with an electric mixer until soft peaks form.

Gently fold the mocha mixture into the whipped cream using a spatula.

In a clean bowl, beat the egg whites with a pinch of salt until stiff peaks form.

Carefully fold the beaten egg whites into the mocha mixture in two additions.

Transfer the mousse into individual serving cups or bowls and refrigerate for at least 2 hours until set.

Before serving, garnish with chocolate shavings or a dusting of cocoa powder.