Boil 2 cups (500 ml) of the whole milk with the vanilla pods and leave to infuse for 20 minutes. Strain this infusion through a chinois*. Sieve the flour and powdered milk together. Add the vanilla infusion to one third of the superfine sugar and bring to a boil. Mix the egg yolks with the powdered milk and flour and the rest of the sugar. Add the remaining whole milk, a little at a time, mixing carefully. Bring the custard to a boil and cook for 5 minutes, stirring briskly with a whisk, then turn it into a bowl to cool. Add the butter at 120/85°F (50/30°C). Place the custard in an airtight container with a layer of plastic on the surface (to prevent a skin from forming) and store in the refrigerator.