A few hours before serving (or more), make the ganache. The cheesecake should be fully cooled and have started the chilling process before topping.
Chop the semi-sweet chocolate into very small pieces and place in a large, heat-proof bowl.
In a small saucepan, heat the cream while gently stirring until it comes to a simmer for about 1 minute.
Pour the hot cream over the chocolate and let sit for 2 minutes, then whisk until smooth.
Remove the chilled cheesecake from the fridge, unclamp the outside of the springform pan.
Let the ganache cool for a few minutes, then slowly pour the chocolate ganache over top of the cake.
Chop the white chocolate into small pieces, place in a microwave-safe bowl, and microwave on medium power for 30-second intervals until melted.
Drizzle the white chocolate over the cheesecake.
Return to the fridge until ready to serve.