Preheat oven to 180⁰C/ 350⁰F. Line the base of a 25 X 25 cm (10 X 10 inches) cake tin with baking parchment. Melt the butter in a saucepan and cook it very gently over low heat for about 5 minutes until it turns golden. Take the saucepan off the heat, add half the white chocolate and stir until smooth. Leave to cool. In a large bowl, beat the sugar and eggs together until pale and fluffy. Add the flour, salt, vanilla, Matcha powder and the cooled chocolate-butter mix and fold it all in until it’s evenly mixed. Pour the mixture into the tin then scatter the rest of the white chocolate over the top of the mix. Bake for 40 minutes or until risen all over. Leave to cool, then dust with little Matcha powder and icing sugar.