Heat the milk in a small saucepan and cook
over medium-high heat until a few bubbles rise to the surface. Stir in the macha and the lime zests and let cool for about 5 minutes.
Meanwhile, beat together the egg yolks and sugar until light. Pour about a quarter of the milk into this mixture, then pour the sugar-egg mixture back into the milk and stir. Add the cream and let rest, refrigerated, overnight.
Preheat the oven to 325 degrees. Pour into 4 6-ounce ramekins (or 6 4-ounce ramekins) and place the ramekins in a baking dish; fill the dish with water halfway up the side of the dishes and cover with foil.
Bake for 30 to 45 minutes, or until the center is barely set. Cool or chill. When you're ready to serve, cover the top of each custard with a thin layer of sugar and caramelize it with a torch (or run it under a broiler). Serve immediately.
