A silky cheesecake made with cream cheese and sour cream -- and featuring melted bitter-sweet chocolate, cocoa powder, plus a chocolate-wafer crust -- is a lesson in decadence.
Preheat oven to 350 degrees. Stir together cookie crumbs, melted butter, and 3 tablespoons sugar in a medium bowl. Press crumb mixture firmly onto bottom and up 1 inch on sides of a 9-inch springform pan. Bake until set, about 10 minutes. Transfer to a wire rack, and let cool. Reduce oven temperature to 300 degrees.
Melt chocolate in a medium heatproof bowl set over a pan of simmering water, stirring, about 2 minutes. Let cool.
Put cream cheese in the bowl of an electric mixer fitted with the paddle attachment; mix on medium speed until fluffy, about 3 minutes. With mixer on low speed, add remaining cup sugar in a slow, steady stream. Add salt; mix until combined. Add sour cream, cocoa, vanilla, and melted chocolate; mix until combined. Add eggs, one at a time, mixing each until just combined (do not overmix). Pour cream cheese filling over crust.
Bake until set but still slightly wobbly in center, 50 to 60 minutes. Let cool completely in pan on rack. Refrigerate, uncovered, 6 hours or overnight. Before unmolding, run a knife around edge of cake.
This cheesecake, once set, can be wrapped in plastic and refrigerated for up to 3 days. Let stand at room temperature for twenty minutes before serving.