Brush the box or bundt pan well with butter and sprinkle with flour (or spray with baking spray).
Separate 3 eggs.
Beat 3 egg whites with a pinch of salt and 125 g sugar until stiff.
Beat 2 eggs and 3 egg yolks with 125 g sugar until frothy.
Gradually stir in the softened butter. Sift the flour and baking powder, mix well and mix with the egg yolk mixture.
Melt the chocolate (in a double boiler or in a microwave) (heat to ~ 45 ° C).
Mix half of the dough with Matcha tea powder and then carefully fold in half of the egg whites using a spatula (or a wooden spoon with a hole in the middle).
Mix the melted chocolate with the other half of the batter; then carefully fold in half of the egg whites with a spatula.
Place the two doughs alternately next to and on top of each other in the pan.
Hit the mold against the table a few times to avoid air holes between the dough.
Bake the cake for 7 minutes at 200 ° C, then for 35-45 minutes at 180 ° C. Make a chopstick test, as the baking time depends on the oven and pan size!
Carefully turn over and dust the still warm cake with powdered sugar (using a sieve).