1. Preheat oven to 350°F (180°C) and line a 6-inch cake pan with parchment.
2. Melt white chocolate and butter in a double boiler until smooth.
3. Stir in matcha powder until no lumps remain, then gradually add milk.
4. Whisk egg yolks with sugar until pale, then mix in the chocolate-matcha mixture.
5. Sift in cake flour and salt, folding gently to combine.
6. Beat egg whites until stiff peaks form, gradually adding sugar.
7. Fold meringue into the batter in three additions, keeping mixture airy.
8. Pour into prepared pan, smooth the surface, and bake 15 minutes at 350°F.
9. Reduce oven to 325°F and bake 20 minutes more; center should still wobble.
10. Cool completely for 4 hours before slicing; dust with sugar before serving.