Preheat your oven to 325°F (160°C).

In a medium bowl, combine the chocolate graham cracker crumbs, melted unsalted butter, and half of the granulated sugar. Mix until evenly moistened and press into the bottom of a 9-inch springform pan.

Bake the crust for 10 minutes until slightly golden. Remove from oven and let it cool.

In a large mixing bowl, beat the room-temperature cream cheese until smooth, then add the remaining sugar and vanilla. Mix well, then add eggs one at a time, and finally mix in the sour cream until smooth.

Pour the cheesecake mixture over the cooled crust and bake for about 40 minutes. Let it cool to room temperature, then refrigerate for at least 4 hours or overnight.

In another bowl, mix softened butter and remaining sugar until light and fluffy. Gradually sift in cocoa powder, mixing well, and then add eggs one at a time and vanilla.

Whip the heavy cream until stiff peaks form and then gently fold it into the chocolate mixture. Spread this over the chilled cheesecake layer.

Cover with plastic wrap and chill for an additional 1-2 hours.

Before serving, top with whipped cream and sprinkle with chocolate shavings or cocoa powder.