Break up the chocolate bar into small pieces and place it into a microwave safe bowl. Melt the chocolate in 30-second intervals, stirring after each interval, until it's melted and smooth.
Mix the cocoa powder into the chocolate until the mixture is smooth, then set it aside.
Add the cream cheese, sugar, and salt to a large mixing bowl. Use a hand or stand mixer to beat the mixture on medium speed for 3-4 minutes, until the mixture is fluffy and smooth.
Pour the melted chocolate mixture and into the cream cheese mixture and add in the sour cream and vanilla. Beat until well combined.
While the mixer is running, slowly pour in the heavy whipping cream. Continue to beat the mixture on high speed for 1-2 minutes, until it thickens.
Take the springform pan out of the freezer and pour the chocolate cheesecake batter over the crust, smoothing it into an even layer.
Cover the pan with plastic wrap and place it in the fridge for at least 4 hours to firm up.