Mock Apple Mixture: Combine sugar, water and cream of tartar in large heavy saucepan. Bring to boil on medium heat, stirring until sugar is dissolved. Remove from heat; stir in salt. Cool slightly. Whisk in butter, lemon juice and vanilla.
Combine crushed crackers, cinnamon and lemon zest with electric mixer fitted with paddle attachment. Add sugar mixture; blend until combined. Transfer mixture to a storage container. Refrigerate at least 10 min. or until ready to use.
For each milkshake: Blend 10 oz. ice cream and 4 oz. Mock Apple Pie Mixture in blender until well combined. Serve in a tall glass topped with 1 Tbsp. crushed RITZ Crackers.