Preheat oven to 350F and grease or line a 9" square pan with parchment paper.
Sift the flour and salt in a separated bowl and set a side.
Chop the chocolate and melt it in the microwave or on top of a boiling water.
Set aside.
In a bowl of a standing mixer, whip the butter and sugar and vanilla sugar until lighter in color and fluffy.
Stir in the egg yolks following the melted chocolate.
In a separate bowl, whip the egg white to a medium peak about 1 and a half minutes.
Fold in about 1/3 of the flour following 1/2 the egg whites. Repeat with the remaining flour and egg whites.
Pour the batter in the prepared pan and smooth with a spatula or the back of a spoon.
Bake in the lowest rack for about 20 minutes or from the center of until a skewer comes out dry from the center of the cake.
Allow the cake to cool completely. Gently remove the cake from the pan dust it with the Matcha Powder and slice to 16 pieces.
Make sure to add the Matcha right before serving. With time, the moist in the cake dissolve the Matcha and the green color disappears.