1 ½cups(340g)butter, cut into 24 pieces and softened at room temperature (I use salted butter)
In a medium bowl, combine the sugar, flour, cornstarch and salt. Slowly whisk in the milk until the mixture is smooth. Place a fine-mesh strainer over a medium saucepan and pour the milk mixture through the strainer into the saucepan. Cook the mixture over medium heat, whisking constantly, until the mixture boils and is thick enough that it starts to become difficult to easily whisk. This could take anywhere between 5-10 minutes, depending on your stove, heat, etc. It should bubble quite a bit at the end (be careful of the splatters) and thicken considerably.
Transfer the mixture to a clean bowl and cool to room temperature – this is extremely important! If it is even slightly warm, the frosting won’t beat up properly. I refrigerated my initial mixture overnight. If you do this, make sure to pull it out in time to let it warm back up to room temperature. If you try to proceed with the rest of the recipe and the mixture is too cold, the butter won’t absorb into the frosting like it should.
Once the frosting is completely cooled to room temperature (it should have no hint of warmth at all!), beat the mixture with the vanilla on low speed until it is well combined, about 30 seconds (a stand mixer will work best for this). Add the butter, one piece at a time, and beat the frosting until all the butter has been incorporated fully, about 2 minutes. Increase the speed to medium-high and let the mixer work it’s magic. Beat the frosting for five minutes, until it is light and fluffy. Let the frosting sit at room temperature until it is a bit more stiff, about 1 hour. I suspect if you chill it for an hour or so, it would be stiff enough to actually pipe with instead of frosting with a rubber spatula.
Plan Ahead: this frosting takes a bit of advance planning since the frosting needs time to cool so plan ahead! Frosting: originally, I put a note on here that was included in the original recipe that the frosting keeps in the refrigerator and then can be rewhipped, but I had to delete that note because every time I have tried it, it has been a major bust. After I have refrigerated it and rewhipped, it separates and never becomes satiny and delicious again. You may find different results but in good faith I have to say that in my experience, this frosting is best made and used fresh. Piping the Frosting: I pipe a lot of cakes for my kids’ birthdays (think: Spiderman covered in Wilton stars) and although I haven’t tried it, I highly suspect that an hour or so of chilling time will lend this frosting nicely to being piped. When I try it, because I will, I’ll update the recipe with my notes.Chocolate Version: add 1/4 cup cocoa powder to the sugar, flour, cornstarch, salt mixture in the first step. Once the frosting has been mixed for five minutes and is light and fluffy, mix in 3 ounces of semisweet chocolate that has been melted and completely cooled to room temperature. This makes a lighter-colored chocolate frosting, but the chocolate flavor is deep and terrific. I increased the melted chocolate to 4 ounces when I made it and may even do 5 ounces next time.