In a medium saucepan over medium heat, combine the milk, 1/4 cup / 50g of the sugar, salt, and vanilla; bring to a gentle boil; and then remove from the heat.
In a medium bowl, whisk together the cornstarch and remaining 1/4 cup / 50g sugar. Add the egg and egg yolks and whisk into a smooth paste.
Whisk enough of the hot milk, a little at a time, into the egg mixture until it is warm to the touch. Once the eggs are warm, pour the mixture back into the remaining milk in the pan.
Return the saucepan to medium-high heat and bring the mixture to a boil; continuously whisk for 3 minutes to thoroughly cook the corn starch.Pro Tip: The pastry cream will get very thick almost immediately, but it is important to continue to cook for the full 3 minutes or the pastry cream will separate once it cools. Remove from the heat, stir in the mascarpone. Using a rubber spatula press the mixture through a fine-mesh sieve, into a large bowl and cover with plastic wrap, pressed directly onto the surface, to prevent a skin from forming.
In a large bowl, add enough ice cubes and cold water to create a bath for the container to sit in without the water breaching the sides.
Set the container in the ice bath or place in the freezer until chilled, about 15 minutes or until room temperature.