Preheat oven to 350°F. Sprinkle chips on bottom of crust.
In medium saucepan over low heat, melt margarine, add cocoa; stir till smooth. Add sweetened condensed milk and water. Stir with whisk until well blended. Stir in egg and vanilla. Remove from heat. Pour into crust.
Bake 30 minutes or until edges are set (center will form when cooling). Cool slightly. Garnish with whipped topping. Serve warm or at room temperature.