In a medium saucepan, heat the milk, cream, vanilla, salt, and 1/2 cup of the sugar over medium heat until simmering.
Meanwhile, in a medium bowl, whisk together the remaining 1/4 cup sugar, egg, egg yolks, and cornstarch until smooth. Once the milk mixture is simmering, slowly pour about 1/2 cup of the hot milk into the egg mixture, whisking constantly. Then, pour the entire mixture back into the saucepan and whisk constantly over medium heat until it is very thick, about 5 minutes.
Pour the custard into a heat-safe bowl through a mesh siv to remove any lumps. Cover the custard with plastic wrap placed directly on top of the mixture, and chill for several hours until completely chilled and firm.
Heat the heavy cream in the microwave in a medium bowl until boiling. Stir in the chocolate chips, vanilla, and salt until the chocolate has completely melted and the mixture is thick and smooth.
First, place one of your yellow cake layers on your plate. Then, spoon the cold custard on top of this layer, and smooth out with a knife or spatula until smooth. The layer of custard will be fairly thick, and should reach all the way to the edges of your cake.
Then, gently place the next yellow cake on top, and press gently to adhere it to the custard. Use a spatula to smooth out any custard that starts to bulge out slightly.
Pour your still-warm ganache over the top of the cake, and smooth it out so that it drips down the edges slightly. Chill the entire cake for at least an hour before serving!