In a large, chilled mixing bowl add the sweetened condensed milk and using the whisk attachment, whip for a minute to lighten and incorporate air.
Add the heavy whipping cream and beat on High until stiff peaks form.
Add the Matcha green tea and vanilla. Whip until fully incorporated.
Transfer to an insulated ice cream storage tub or lined meatloaf pan (See Note 1). Four hours will make a firm, chilled, mousse-like ice cream. Freezing overnight makes for a firmer ice cream.
Set out for at least 15 minutes prior to scooping. Will keep in the freezer for up to one week.
Notes
If you don't have the insulated ice cream storage tub (see link above) I would use a meatloaf pan lined with parchment and cover loosely with plastic wrap, then foil to help freeze.