Heat the chocolate and espresso in a water bath.
Once the chocolate melts, add it to the separated egg yolks that you've mixed with the butter.
Stir until homogeneous, and then add the pepper.
Leave the mixture to cool a little. In the meantime, beat the egg whites to snow with the sugar.
Add them to the cooled chocolate mixture but do so carefully so they don't deflate.
Distribute the mousse into cups and put them in the refrigerator to cool.