In a large bowl, add 1 ¼ cup heavy whipping cream, powdered sugar, and ¼ cup cocoa powder, and start mixing on low speed until combined. Slowly increase speed to high until stiff peaks form. Place in the refrigerator.
Add the chocolate into a microwave-safe bowl and heat in 30-second increments, stirring in between, until fully melted and smooth. Set aside to cool.
In a large bowl, cream the cream cheese for 4-5 minutes or until smooth and there are no lumps. Pour in the cooled chocolate and mix until combined.
Add the sugar and vanilla and mix until combined.
Add in the 4 Tbsp heavy cream last and mix until just combined.
Pour the cream cheese mixture over the frozen crust and smooth.
Place in the refrigerator and chill overnight to make sure the cheesecake is fully set.
Remove the cheesecake from the springform (I find the best way to do this with no-bake cheesecakes is to run hot water over the OUTSIDE of the springform pan. After about two paces, the cheesecake should melt just enough to release easily from the springform).