|360g||cold unsalted butter|
Makes about 800g.
To make the roux, put the milk into a saucepan over a medium heat until steaming.
Meanwhile, mix the cornflour/cornstarch with 100 ml/1⁄3 cup plus 1 tablespoon cold water. Add this to the steaming milk and whisk vigorously! Heat, stirring continuously, for 2–3 minutes until the roux is bubbling a little and thickened and the cornflour/cornstarch has cooked out. Pour the roux into a shallow dish or tray. Pop clingfilm/plastic wrap on top and let it cool and solidify.
To finish the buttercream, put the cold butter and sugar into a mixing bowl. Add the cold solidified roux and beat with a hand-held electric whisk (or use a stand mixer with the paddle attachment). The mixture will start to break up and look split and slushy, but keep beating for 4–6 minutes, scraping down the sides of the bowl occasionally. The buttercream will turn into something that resembles whipped double/heavy cream, but be sure not to overmix as it can split.