Measure chocolate into a heatproof bowl.
Melt it over a double boiler or in a microwave. If using chocolate block, chop it into small pieces for quick melting.
To melt the double boiler way, fill up some water into a small pot. Place the bowl of chocolate over the pot, making sure the water does not touch the bowl. For this purpose, it helps to use a pot that is smaller than the bowl.
Let the steam from the water warm up the bowl and melt the chocolate. Stir occasionally to expedite the melting.
To melt using the microwave, place the chocolate in a microwave safe bowl. Heat it up at short intervals of 30 seconds on medium or high until the chocolate is fully melted.
Set the chocolate aside to cool completely to room temperature (see Note 1).
In a separate bowl, whip the cream until soft peaks from (see Notes 2 & 3). Set aside in the refrigerator while waiting for the chocolate to cool down.
In another separate bowl, measure the mascarpone cheese and powdered sugar. Cream both until well incorporated.
Once the chocolate has cooled down to room temperature, add it to the mascarpone cheese mixture. Add in the vanilla extract and whip until well combined.
Next, remove the whipped cream from the fridge. Set aside 2 spoonful of the cream. Keep it refrigerated.
From the remaining cream, add approximately 2 spoonful into the chocolate mascarpone mixture. Fold until well combined. Repeat with 2 more spoonful of cream before folding in all the remaining whipped cream. Mix until well combined.
Transfer the mascarpone chocolate mousse into a large piping bag. Snip the tip and pipe it into prepared mousse cups. To level the tops, place a folded kitchen towel on your kitchen countertop or table and tap the mousse cups gently on cloth to level the mascarpone chocolate mousse.
Next, grate some white chocolate onto each of the chocolate mousse cups.
Transfer the reserved whipped cream into a piping bag fitted with a large star tip and pipe a large whipped cream star in the center of each mascarpone chocolate mousse cup. You can also just add a dallop of the cream onto the mousse with a teaspoon.
Chill the mousse for at least 1 to 2 hours before serving to allow it to set. Keep the mousse chilled at all times, until ready to serve. This mascarpone chocolate mousse is best served chilled.