1. Make the cake according to the package directions, baking it in a 9 by 15 inch baking pan. Let cool completely.
2. To make the filling, whisk the flour into the half and half in a saucepan. Place over medium heat and cook, whisking constantly, until it becomes very thick. This will take a few minutes. Allow to cool completely.
3. Beat the butter, sugar, and vanilla together for the filling until fluffy. Beat in the cooled flour mixture thoroughly until light and fluffy. Spread this mixture onto the cooled cake.
4. In a large bowl, beat together powdered sugar, butter, and melted chocolate until smooth. Add milk, boiling water and vanilla extract and beat for about five minutes until very fluffy. Spread icing over cream filling.
5. Place in the refrigerator to set before cutting into squares to serve. Store any leftovers, covered, in the refrigerator.