For a special treat, serve this rich dessert with gourmet coffee. If you prefer a milder liqueur flavor, use a little less Irish cream and add whipping cream or milk to make up the difference.
Preheat oven to 325 degrees F. For crust, in a medium bowl, combine crushed chocolate wafers, melted butter, and cinnamon; toss gently to mix. Press mixture onto the bottom and up the side of a 9- or 10-inch springform pan. Set aside.
For filling, in a large bowl, combine cream cheese, sour cream, sugar, and melted chocolate. Beat with an electric mixer on medium to high speed until smooth. Using a wooden spoon, stir in eggs just until combined. Stir in liqueur, whipping cream or milk, and vanilla.
Pour filling into the crust-lined pan. Place springform pan in shallow baking pan. Bake for 50 to 60 minutes or until center appears nearly set when gently shaken.
Cool in springform pan on a wire rack for 15 minutes. Using a small sharp knife, loosen from side of pan. Cool for 30 minutes more. Remove side of springform pan. Cool for 1 hour. Cover and chill for at least 6 hours or up to 24 hours. Makes 16 servings.
If using a 10-inch springform pan, use 1-3/4 cups crushed chocolate wafers (about 21 cookies) and 7 tablespoons butter, melted, for the crust.