Over low-medium heat, put together chocolate and butter until melted and smooth. Set aside.
Add and stir the yolks into the chocolate mixture one by one.
In a separate bowl, beat and whisk egg whites with an electric stand mixer until stiff peaks form.
To achieve a softer texture, pour 1/3 of the whites into the melted chocolate.
Combine and stir the mixture gently with a spoon.
To serve, transfer the mousse into preferred glasses, bowls or separate ramekins.
Store in the fridge for 2-3 hours before serving.