Melt chocolate with butter over a bain-marie and mix very well until incorporated.
Whip yolks with (50g) sugar until pale yellow and in the ribbon stage. Set aside.
Whip whites with sugar (44g) until it forms soft peaks.
With a spatula, gently fold the yolk mixture with the chocolate until incorporated.
Fold the whites into the chocolate mixture. Start with a small amount of whites. Use a whisk to fold into the chocolate, gently folding from the bottom of the bowl to the top until incorporated.
Ladle the mousse into four 8oz ramekins or one big sharing bowl.
Let the mixture set for 4 hours in the refrigerator before serving with strawberries.