Fill half of a pot with water and bring it to a simmer. Chop chocolate into small pieces for easier melting. Put the chocolate, espresso, salt and peperoncino in a bowl and set it on top of your pot of hot water to create a double boiler. Stir occasionally until it is completely melted. Once the chocolate is melted, set the mixture aside and let it cool to room temperature.
In a cold bowl, add heavy cream and whip until it is stiff. Place in the refrigerator and allow to cool.
Separate eggs. Combine the yolks with the sugar and whisk until the mixture is lightened. Add the melted chocolate to the lightened yolks and mix well.
Add whipped cream to the chocolate mixture in small batches, folding gently until it is combined.
In a separate bowl, add a pinch of salt to the egg whites and whip until it forms soft peaks. Add the egg whites to the chocolate mixture and fold until combined.
Spoon or pipe your mousse into a bowl vessel and cover with plastic wrap. Chill in the refrigerator for up at least six hours (or overnight). Serve alone or with whipped cream.