Preheat oven to 350° F
Grease an 8″x8″ or 8″x6″ baking pan with butter; set aside.
In a medium bowl, combine Group A Ingredients; walnut, flour, chocolate, baking powder, matcha and salt.
In a large bowl, whisk sugar and eggs together until pale and thickened, 1 to 2 minutes. Whisk in butter and vanilla, then add flour mixture (group A) and stir together just until combined.
Transfer to pan and bake until just set in the middle and golden brown around the edges, about 30 minutes. Set aside to let cool, then cut and serve.