Add the milk powder, sugar and salt to the Thermomix bowl. Mix for 3 sec/speed 5 to combine. Scrape down the sides of the mixing bowl with a spatula.
Add the butter and water. Heat for 10 min/90°C/speed 3 with the simmering basket on top of the lid instead of the measuring cup. The mixture will be smooth and slightly thickened (like custard), it will thicken more as it cools.
Pour the sweetened condensed milk into a jar or airtight container. Leave it to cool down for 20 minutes then transfer to the fridge.
Notes
* This recipe can be doubled, just increase the cooking time by 3 minutes so it's not grainy.Storage: Keep in an airtight container in the fridge for up to 2 weeks. If it separates, stir before using. It also freezes well for up to 3 months - thaw overnight in the fridge and stir well until smooth.