Line a half-sheet baking pan with plastic wrap, make sure the sides of the tray are also covered. Set aside.
Pour 1 ½ cup of milk into a large heavy bottomed saucepan. Add salt and ½ cup sugar. Whisk together. Place over medium-high heat and bring to a boil. Stir occasionally to make sure the milk does not burn along the sides or bottom of the pan.
Meanwhile, in a medium bowl, whisk together cornstarch and ¼ cup sugar. Add in remaining ½ cup of milk and yolks. Whisk until smooth.
When the milk has boiled, slowly add a little bit of the milk into the egg mixture. Whisk constantly to distribute the heat. Continue to pour and whisk until only a couple tablespoons of milk are left the saucepan. Then pour the egg-mik mixture back into the saucepan.
Whisk constantly over medium heat until the custard has thickened. Keep whisking and let the custard come to a boil to cook the cornstarch out. Remove from heat. Whisk in butter, whisk until smooth. Add vanilla and whisk until combined.
Pour the hot pastry cream into the prepared plastic wrapped sheet pan. Spread evenly. Cover with plastic wrap and freeze for 10-15 minutes until the pastry cream has cooled to room temperature. Remove from freezer and continue to cool in refrigerator until ready to use.