Preheat oven to 325 degrees F. In a medium bowl, combine the crumbs, 1/4 Cup sugar and the margarine and press the mixture frimly onto the bottom and side of a 9 inch pie plate; set aside. In a small saucepan over low heat, melt 3 squares of chocolate; set aside. In a medium bowl, with the aid of an electric mixer set at medium speed, beat the cream cheese and the remaining sugar until the mixture is creamy. Beat in 1/2 Cup of sour cream and the eggs until the mixture is smooth. Blend in the melted chocolate and vanilla extract; pour the filling into the prepared crust and bake the cheesecake for 35 to 40 minutes or until the filling is slightly puffed and the center is set. Blend the remaining sour cream and brown sugar and spread the mixture over the cheesecake; bake for an additional 5 minutes. Cool the cheesecake to room temperature. Melt the remaining chocolate and drizzle it over the cheesecake; chill for at least 4 hours.