Step 1

To make the pastry cream: In a medium-sized saucepan, stir together 1 1/4 cups (283g) of the milk, the sugar, and salt. Bring to a simmer over medium heat, stirring to dissolve the sugar.

Step 2

, Meanwhile, in a medium bowl whisk the cornstarch, flour, and egg yolks with the remaining 1/4 cup (57g) milk.

Step 3

, Whisk about half of the hot milk mixture into the egg yolk mixture to temper the yolks. This keeps them from turning to scrambled eggs when you add them to the simmering milk.

Step 4

, Pour the egg/milk mixture back into the remaining simmering milk. Bring to a boil, stirring constantly with a whisk, until the mixture thickens and bubbles not just at the edges but all the way across its surface.

Step 5

, Remove from the heat and strain through a fine strainer into a bowl set in an ice bath (a bowl of ice water). Stir in the 2 tablespoons (28g) butter and the vanilla.

Step 6

, Cover the pastry cream with a piece of plastic wrap, making sure it touches the top of the cream to prevent a skin from forming, and refrigerate until cool.

Step 7

, To finish the buttercream: About 30 minutes before you want to finish the buttercream, remove the chilled pastry cream from the refrigerator and stir it with a spatula to smooth it out and take the chill off. The closer the pastry cream is to room temperature, the easier it will whip into the butter. Look for the pastry cream and butter to be about the same temperature and consistency.

Step 8

, In a stand mixer fitted with the paddle attachment, beat the 24 tablespoons (340g) butter at medium to high speed until smooth and fluffy, about 4 to 6 minutes.

Step 9

, Add the pastry cream to the whipped butter mixture a few tablespoons at a time, beating well after each addition.

Step 10

, Once all the pastry cream has been added, the frosting may look separated or grainy, similar to ricotta cheese. Switch to the whisk attachment and whip on high speed until light and fluffy, scraping down the sides of the bowl as needed. If the bottom of the bowl is cool to the touch, or if the buttercream looks dense or greasy, gently warm the mixture by setting the bowl over a pan of steaming-hot water for a few minutes, just until you see the edges begin to soften/melt. Re-whip until the mixture is glossy and silky smooth, like mayonnaise.

Step 11

, Use the frosting immediately to finish the dessert of your choice. You'll have enough to frost two dozen cupcakes or an 8” or 9” layer cake.

Step 12

, Store any leftover frosting in an airtight container in the refrigerator for several days. To re-whip, bring buttercream to room temperature, then gently warm the mixture by setting the bowl over a pan of steaming-hot water for a few minutes (just until you see the edges begin to soften/melt). Whip until the mixture is glossy and smooth.