First, preheat the oven to 350°F (180°C), or 320°F (160°C) for a convection oven, and line an 8×8 inch (20×20 cm) pan with parchment paper.
Then mix the flour, the cocoa powder, both kinds of sugar, and the salt, and also sift in the Matcha powder.
In another bowl, meanwhile, mix the lukewarm melted butter, the eggs, and the vanilla until smooth.
Fold the two mixtures together, but only until the batter is smooth.
Next, spread it in the pan, sprinkle it with white chocolate, and then marble it with a toothpick.
Bake it for 25-30 minutes, and after the toothpick test, if needed, bake it for another 3-5 minutes.
Finally, let it cool completely in the pan, and only then slice it.