Heat oven to 300°F. In ungreased 13x9-inch pan, combine all crust ingredients; mix well. Press firmly in bottom of pan.
Beat cream cheese until fluffy in large bowl. Add condensed milk, melted chocolate, vanilla and eggs; beat until smooth, scraping down sides of bowl frequently. Pour filling over crust.
Bake at 300°F. for 40 to 50 minutes or until set. Cool 30 minutes.
In small saucepan, melt 2 oz. unsweetened chocolate and 2 tablespoons margarine over low heat, stirring until smooth. Remove from heat. Add water and powdered sugar; beat until smooth.
Spread topping over warm cheesecake. Refrigerate 1 1/2 hours or until cold. To serve, let stand 15 minutes. Cut into squares.