In a medium saucepan, combine the sugar, cornstarch and salt.
Gradually add the milk and blend gently.
Cook over medium heat, stirring constantly until the mixture boils.
Boil the mixture for 1 minute and then remove from the heat.
Place the slightly beaten egg in a small bowl.
Slowly drizzle 1/2 cup of the hot milk mixture into the egg, whisking constantly.
Return the egg/milk mixture to the saucepan and continue to cook over medium heat.
Add the butter and stir until it melts.
Pour the mixture into a small mixing bowl and refrigerate for at least 3 hours or until the mixture is completely chilled.
After the mixture is chilled, add the rum flavoring and beat at medium speed until the mixture is light and creamy, about 2 minutes. Chill until you are ready to frost the cake.
For the Cake:
Grease the bottom of a 9x13 pan.
Preheat the oven to 350 degrees.
In a large mixing bowl, beat the heavy cream until it is thickened and can hold a peak.
Add the eggs, one at a time, beating well after each addition.
Gradually beat in the sugar and rum flavoring.
Add the flour, baking powder, salt and nutmeg. Blend at low speed just until combined, scraping the bowl as needed.
Pour the batter into the prepared pan and spread evenly.
Bake for 45-50 minutes or until the center springs back when you touch it lightly.
Allow the cake to cool completely.
Frost the cake with the whipped rum cream topping.
Sprinkle with additional nutmeg, if desired.
Store in an airtight container in the refrigerator.
The calories shown are based on the cake being cut into 15 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.