Preheat oven to 350°. In a small bowl, mix cracker crumbs, brown sugar, cocoa and salt; stir in yogurt. Press onto bottom of a greased 9-in. springform pan. Place pan on a baking sheet. Bake 10 minutes. Cool on a wire rack. Reduce oven setting to 325°.
In a large bowl, beat cream cheese and sugar until smooth. Beat in pumpkin, sour cream, flour, vanilla and spices. Add eggs and egg yolk; beat on low speed just until blended. Pour into crust. Return pan to baking sheet.
Bake 1-1/4 to 1-1/2 hours or until center is almost set. Cool on a wire rack 10 minutes. Loosen sides from pan with a knife. Cool 1 hour longer. Refrigerate overnight, covering when completely cooled.