Preheat oven at 180ºC. Grease 2 baking tins with some butter and dust some refined flour.
Cream together butter and castor sugar with an electric blender till soft and fluffy. Add condensed milk and mix well.
Take a green tea bag in a bowl, add 2-3 tablespoons warm water and set aside. Add green tea decoction to butter cream mixture, mix and whisk well using the electric blender. Add edible green color and vanilla essence and mix well.
Sift together refined flour, baking powder and baking soda into the butter cream mixture and fold in well.
Fill both the greased baking tins with the batter. Place the baking tins on a baking tray, place the baking tray in the preheated oven and bake for 20-22 minutes. Remove from oven and demould.
To prepare icing, heat cream in a non-stick pan till warm.
Put white chocolate in another bowl, add warm cream and mix well till chocolate melts. Add whipped cream and fold in gently. Refrigerate till set.
Trim off a thin slice from the top of each cake to level the surface and reserve the slices. Halve both the cakes horizontally.
Place one half cake on a worktop, brush some sugar syrup, spread some icing mixture and place another half over it. Repeat the process till all the halves are used. Level the sides of the cake and cover with some icing mixture.
Put the remaining icing mixture in a piping bag fitted with star nozzle and pipe out rosettes on the outer edge of the top of the cake. Crumble the reserve cake slices and place them in the center. Decorate with the edible golden balls.