Step 1

Place the milk in a saucepan over medium heat and bring to a simmer.

Step 2

Meanwhile, in a medium mixing bowl whisk together the sugar, cornstarch, and eggs until well combined.

Step 3

Slowly pour about half of the milk into the egg/cornstarch mixture while whisking continuously. It can be helpful to nestle the mixing bowl in a damp kitchen towel to stabilize the bowl so you can whisk with one hand.

Step 4

Pour the milk/egg mixture back into the pot with the remainder of the milk and cook over medium/low heat, whisking continuously, until the mixture thickens and comes to a boil. Once the mixture is boiling, turn the heat to low, and continue whisking vigorously for 2 minutes. The mixture will look lumpy at first but will smooth out as it cooks. This will be a very thick custard.

Step 5

Remove the custard from the heat and whisk in the vanilla extract. Pour into a shallow dish and press a piece of plastic wrap right on the surface of it. Chill for 1 hour or up to overnight. If you chill for longer than an hour remove it from the refrigerator enough for it to come up to about 50 F (10 C). The custard will become a solid mass as it cools.

Step 6

In the bowl of the stand mixer fit with the paddle attachment, beat the butter on medium high speed for about 5 minutes until light and fluffy.

Step 7

Add the custard to the mixing bowl one spoonful at a time while the mixer is still running. Scrape down the sides of the bowl periodically as needed. Continue mixing until all of the custard has been added.

Step 8

Switch to the whisk attachment and whip on medium/high speed for about 4 minutes until light and fluffy.

Step 9

Use the frosting immediately or store covered in the refrigerator for up to 3 days. Re-whip before using.