Separate the eggs into two bowls: one with egg yolks and one with egg whites. Set aside to let come to room temperature.
Warm the milk in a small saucepan or double boiler over medium-low heat until just steaming (not boiling). Add the chocolate and stir until chocolate is completely melted and smooth. Remove from heat and let cool slightly.
In a clean, dry bowl of a stand mixer fitted with a whisk attachment*, add the egg whites. Whip on medium speed until foamy, then add the salt.Increase speed to medium-high and whip until stiff peaks form. Be careful not to overwhip! The whipped egg whites should still look shiny and silky, not coarse or chunky (overwhipped).**
In a large bowl, combine the melted chocolate mixture, egg yolks, vanilla extract, and cayenne pepper (optional). Stir to combine.
Add ⅓ of the egg whites to the mixing bowl and fold to incorporate, scraping around the edges of the bowl. This will make it easier to incorporate the remaining egg whites. Gently fold in the remaining egg whites until almost fully incorporated. Add cacao nibs during the last part of your folding.
Divide the mousse among 4 ramekins. Cover with plastic wrap and chill for at least 2 hours. When ready to serve, remove the plastic wrap, top with dairy-free whipped cream, fresh berries, and chocolate shavings, then serve!
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Notes
*You do not need a stand mixer to make this recipe. You can also whip the egg whites in a large bowl with a hand-held electric mixer. **If you have overwhipped your egg whites, it's ok! They will still taste good, but your chocolate mousse will be thicker and less smooth than normal. You can either try again with fresh egg whites, or continue with the recipe as written (it will still work out!).