Separate the egg yolk into a mug, beat it, then set it aside.
In a medium saucepan, combine the sugar, flour, cornstarch, and salt.
Gradually stir in the milk.
Over medium heat, heat and stir until the mixture thickens and boils.
Stirring constantly, cook for 2-3 minutes longer. Remove from heat.
Temper the egg yolk. (While whisking constantly, slowly spoon the hot mixture into the egg yolk.)
Then, whisking constantly, slowly pour the egg yolk mixture into the saucepan.
Whisking constantly, over med/low heat, bring just to a boil. Remove from heat and stir in the vanilla.
Pour into a bowl, cover the entire surface with plastic wrap (prevents a scum from forming) and cool.
While the custard is cooling, beat the cream into whipped cream.
When the custard is cooled, beat it smooth then fold in the whipped cream.