Heat the oven to 325°F (165°C). Grease the pan. Mix cookie crumbs and butter, press across the base, and bake for 10 minutes.
Melt semisweet chocolate gently and let it cool until barely warm. Place a roasting pan of hot water on the lower oven rack.
Beat cream cheese and sugar on low until smooth. Mix in sour cream, sifted cocoa, and vanilla, scraping away any hidden cheese lumps.
Blend in cooled melted chocolate. Add eggs individually on low, stopping as soon as they disappear into the filling.
Pour into the crust and smooth the top. Bake above the water pan for 60–75 minutes, until the outside is set and the center gently wobbles.
Turn the oven off and leave the door slightly open for 1 hour. Keep the cake inside so it cools gradually.
Cool on a rack for about 1 hour, then chill for at least 6 hours. Cover once cold without pressing wrapping against the surface.
Release the ring after loosening the edge with a thin knife. Cut chilled slices with a warm dry knife and wipe between portions.
