To Make Filling: In a heavy saucepan over medium heat, cook condensed milk, brown sugar, butter and corn syrup. Stir and bring to a boil; stir continuously for at least 5 minutes. Remove from heat, then slowly beat with a spoon until mixture starts to thicken; will take about 2 minutes. Be sure not to over beat or it will harden too much. Pour mixture into pie shell; cover and refrigerate for about an hour. Filling ingredients may be halved to make for a less rich pie.
To Make Topping: In a medium saucepan, cook chocolate and butter over low heat until melted together. Stir in condensed milk; mix well. Remove from heat; allow to cool just enough so that yolk won't cook when added, but not so cool that the chocolate hardens; cooling time will be about 3 minutes. Stir in egg yolk to chocolate mixture; spread over cooled filling. Place pie back in refrigerator and cool for another hour.