Line a half-sheet baking pan with plastic wrap, making sure the sides of the tray are also covered. Set aside.
Pour 1 ½ cup of milk into a large heavy bottomed saucepot. Add 2 Tbsp (25 grams) of sugar. Stir together. Place over medium-high heat and bring to a boil. Stir occasionally to ensure milk does not burn along the sides or bottom of pan.
Meanwhile, In a large bowl, whisk together cornstarch and remaining ½ cup + 2 Tbsp (125 grams) sugar. Add egg yolks and remaining ½ cup milk. Whisk until smooth.
Once milk has a come to boil, reduce heat. Slowly add a little bit bit of the hot milk (about ⅓ cup at a time) into the egg mixture. Whisk constantly to distribute to heat. Continue until only a couple tablespoons of milk are left in the saucepot. Then, pour all the egg-milk mixture back into the saucepot.
Whisk constantly over medium heat until the custard has thickened. (Use a whisk or a spatula) Keep stirring, ensuring to scrape the bottom and sides of the pot. Let custard come to boil to cook out the cornstarch flavor. Remove from heat.
Whisk in butter until melted and distributed. Add vanilla and whisk until combined.
Pour the hot pastry cream into the prepared plastic wrapped sheet pan. Spread into an even layer. Cover the cream directly with plastic wrap. Freeze for 10-15 minutes until the pastry cream has cooled to room temperature.
Remove from the freezer. Continue to cool in refrigerator until ready to use.