In a large bowl, melt chocolate with butter, set aside.
In a second bowl: whip the cream and maple syrup to soft peak, keep in fridge.
Separate the egg yolk from egg whites.
In a third bowl, place the egg whites and start whisking.
Add gradually the sugar to form a light meringue.
Mix the egg yolk into the melted chocolate.
Combine all three bowls.
First add the egg white into the chocolate, keep it as airy as possible.
Then add the whipped cream in the chocolate, slowly, to keep a light consistency.
Pour the mousse in the glasses and set in the fridge.