Crust: combine crushed crackers and cinnamon. Stir in butter. Press mixture onto bottom and about 2 inches up the sides of an ungreased 8 or 9 inch springform pan. Set aside.
In a large mixing bowl beat cream cheese, sugar, flour, and vanilla with an electric mixer until combined. Add eggs and egg yolk all at once, beating on low speed just until combined. Stir in milk. Separate into two equal amounts. Add melted chocolate to one half and stir.
Pour chocolate filling into the crust-lined pan and then carefully pour the plain filling on top. Use a spatula to gently swirl fillings
Place in a shallow baking pan in the oven. Bake at 375 degrees for 45 to 50 minutes for the 8-inch pan or 35 to 40 minutes for the 9-inch pan. Center should appear nearly set when gently shaken.
Cool on a wire rack for 15 minutes. Loosen crust from the sides of the pan. Cool for another 30 minutes. Remove sides of pan and cool for an additional hour. Cover and chill for at least 4 hours.