Line the bottom of an 8-inch round pan. Check the oven to make sure the rack is at 1-2 levels below the middle of the oven. Preheat oven to 310 F.
Separate eggs' whites and yolks into different mixing bowls.
Beat eggs' whites until large bubbles presented, then add a pinch of salt and cream of tartar and beat till those bubbles become finer.
Gradually adding sugar (leaving about ¼ to add to the yolks) while beating with your hand mixer until whites become soft peak.
Switch to the yolks bowl, beat them to smooth while adding remaining sugar.
Melt butter, warm milk, and while hot mix with Matcha.
Measure and sift flour and cornstarch to yolks bowl while whisking in one direction until full mix. Stir in the butter/matcha mixture as well.
Spoon ⅓ of the white meringue into yolks, fold mix to combine, and soften up the yolk mixture.
Add remaining white meringue into yolk mixture, carefully folding-mix to combine them without breaking all the aeration.
Pour this batter into lined cake pan, tap a couple time to remove large bubble then bake for 30 minutes.
Once done, transfer to cooling and cool at room temperature before assembling with mousse in the springform pan.