Make the crust: In a bowl, mix flour, sugar, and salt. Drop in cold butter and blitz with a pastry cutter or fingers until the mixture resembles coarse crumbs. Add just enough water to bring it together. Form into a disk, wrap, and chill for 30 minutes.

Roll and bake the crust: Roll the dough to about 12 inches in diameter. Fit into a 9-inch tart pan, trim the edges, and chill 15 minutes. Bake at 350°F (175°C) for 15-18 minutes until lightly golden. Let it cool completely.

Make the filling: In a small bowl, whisk 2 tablespoons of cornstarch with a little milk to make a slurry. In a saucepan, heat cream and milk until just simmering. Remove from heat and whisk in white chocolate until melted and smooth. Stir in the slurry to thicken a bit.

Add color and flavor: Sift in matcha powder and sugar. Whisk until fully incorporated and smooth. Return to a gentle simmer for 1-2 minutes, then remove from heat.

Assemble: Pour the warm filling into the cooled crust. Smooth the top with a spatula. If you like, tap the pan to release any air bubbles.

Chill: Refrigerate 2-3 hours, or until set. Slice and serve chilled or at room temperature.