Place finely chopped white chocolate in heatproof bowl.
In a heavy saucepan, boil the heavy cream over medium-low heat. Turn off the heat when you start to see small bubbles forming around the edge.
Immediately pour the hot cream over the chopped white chocolate. Using a rubber spatula, slowly stir in a circular motion, starting from the center of the bowl, and working out to the sides. Be careful not to add too much air to the ganache. Stir until all of the white chocolate is melted, for about 2 minutes. It may look done after 1 minute of stirring, but keep going to be sure it's emulsified. Let cool slightly.
To assemble: Drizzle on the white chocolate ganache and garnish with the chopped walnuts. Alternatively, you can simply omit the ganache and dust on some confectioners sugar.