In a small pot, bring the milk and half of the sugar to a simmer over medium heat. While the milk heats, whisk the remaining sugar with the cornstarch. Place the eggs in a medium, heat-safe bowl.
When the milk is hot, whisk the sugar/cornstarch mixture into the eggs to fully combine. Gradually add the milk to the egg mixture in a slow, steady stream, whisking constantly. Don't dump the milk in too fast, or you risk scrambling the eggs!
Return the mixture to the pot and cook until the mixture comes to a boil—large bubbles should emerge from the center of the pot. Remove the pot from the heat and mix in the 2 tablespoons of butter.
Transfer the pastry cream to a shallow bowl and cover directly with plastic wrap. Chill the pastry cream in the refrigerator for at least 1 hour.
Transfer the cooled pastry cream to the bowl of an electric mixer fitted with the whisk attachment, and whip for 1 to 2 minutes, to aerate. While the mixer is running, gradually add room temperature butter in 1/2-tablespoon chunks. Continue adding and mixing until all of the butter is added and the buttercream is light and smooth.
Beat in the powdered sugar and vanilla, mix to combine. The buttercream can be used immediately or refrigerated in an airtight container.
I always have three kinds of hot sauce in my purse. I have a soft spot for making people their favorite dessert, especially if it's wrapped in a pastry crust. My newest cookbook, The Book on Pie, is out on November 10th, 2020.