Add the plain flour and baking powder to the Thermomix bowl.
Mix for 10 sec/speed 5.
Transfer the flour to an airtight container and store it in a cool, dark pantry for up to 6 months.
Notes
Ensure your baking powder is fresh: If the baking powder is past its use-by date, your cakes and muffins won't rise properly. To check if it's still active, try this quick test:
Boil some water.
Place ½ a teaspoon of baking powder into a small ramekin or mug.
Pour in about 100ml of the hot water.
If it fizzes energetically, it's active. If it does nothing or fizzes weakly, it's time to replace it.