Melt Chocolate and Butter: Set up a double boiler with hot water, making sure the bottom of the bowl does not touch the water. Place chopped dark chocolate and butter in the bowl and melt together until shiny and glossy. Set aside.
Whip the Cream: Whip the chilled whipping cream to soft peaks. Cover and refrigerate until needed.
Prepare Egg Yolk Mixture: Whisk 3 egg yolks with half (25 g) of the castor sugar. Add 1 tablespoon of cocoa powder and mix. Place the mixture over the double boiler and whisk constantly until it thickens but does not cook. Remove from heat.
Combine Chocolate and Egg Yolks: While whisking, slowly add the melted chocolate and butter mixture into the thickened egg yolk mixture. Continue whisking until fully incorporated. Set aside to cool slightly.
Beat Egg Whites: Beat the 3 egg whites with a pinch of salt to soft peaks. Gradually add the remaining 25 g castor sugar in batches and continue beating to stiff peaks.
Fold Egg Whites into Chocolate Mixture: Fold the beaten egg whites gently into the chocolate mixture in batches, folding in one direction to retain air and fluffiness.
Fold Whipped Cream: Fold about three-quarters of the whipped cream into the chocolate and egg mixture gently in batches, maintaining the airiness.
Chill the Mousse: Spoon the mousse carefully into ramekins without deflating. Cover with cling film and refrigerate for at least a few hours to set.
Garnish and Serve: Before serving, garnish with the reserved whipped cream and chocolate shavings.