In a medium saucepan over low heat, melt dark chocolate with butter, stirring until smooth. Set aside to let cool.
In a large bowl, beat cream cheese until smooth. Beat in sweeteners and vanilla extract, then beat in eggs one at a time, scraping down beaters and sides of bowl as needed.
Beat in cocoa powder and heavy cream until well combined, then beat in melted chocolate until mixture is completely smooth.
Brush sides of springform pan with melted butter, taking care not to disturb the crust. Pour filling into pan and gently shake from side to side to even out. Bake 55 to 60 minutes, or until filling is mostly set but still jiggles just a little in the center.
Remove and let cool 10 minutes, then run a sharp knife around the inside of the pan to loosen. Let cool completely. Once cool, remove sides, cover tightly in plastic wrap and refrigerate at least 3 hours.