In a medium saucepan, whisk together the milk, cream, sugar, cornstarch, milk powder, and vanilla extract until smooth and fully dissolved.
Place the pan over medium-low heat and cook, stirring often, until it begins to gently bubble.
Continue cooking on low to medium-low heat for 45 minutes to 1 hour, stirring frequently to prevent sticking. The mixture should reduce and thicken to a creamy, jammy consistency that coats the back of a spoon.
While it’s cooking, whisk the matcha powder with the water in a small bowl using a W and M motion until completely smooth and lump-free.
Once the milk jam has thickened, remove the pan from heat. Add the matcha mixture and whisk until fully combined and smooth.
Pour the matcha milk jam into a clean jar and let it cool to room temperature. It will thicken slightly as it cools.
Store in the refrigerator for up to one week. It continues to firm up the longer it sits in the fridge, so give it a stir before using if needed!