For the cake: Prepare a 21cm diameter round pan lined with parchment paper at the bottom of the mold.
Sift flour and green tea flour 2-3 times to make it smooth.
Add eggs and sugar into a heat-resistant bowl. Place the bowl on a pot of boiling water (bottom of the bowl does not touch water). Beat the eggs at a small speed and then gradually increase to high speed. Beat until the air bubbles become smaller, and when the egg warms, lift the bowl out of the water. With this method, the eggs will be faster and more stable.
Beat the egg until it becomes thick and smooth, slow down. When you lift the stick, you will see smooth egg flowing down the line like a ribbon.

Divide the flour mixture into 3 parts, sift into the egg. Mix gently in a fold pattern (scrape the dough from the bottom of the bowl and scoop up and fold) after each addition. Be mixed (A).
Heat butter and milk in the microwave for about 30 seconds to melt the butter.
Add 1/3 of the mixture (A) to the well-mixed buttermilk mixture (B). Slowly pour the mixture (B) back into 2/3 of the remaining mixture (A). Mix gently in a gentle fold to avoid air bubbles.

Pour the dough into the mold lined with parchment paper. Tap on the molds a few times to remove air bubbles.
Prepare the oven at 170 degrees Celsius. Bake the cake 17-20 minutes.
Cake cooked to cool, get out of the oven, wrap the food in plastic wrap, the yellow cake face down.
When the cake cools, use a bottomless 18cm diameter mold (mousse ring) to cut the cake. Place the cake on the plate with the cake surface on the bottom.

Prepare a piece of white chocolate mousse:
Soak gelatin in water. Beat the egg whites with high speed and let them float (creating a pointed tip).
Whip 250ml of whipped cream, thick but still soft (don’t overdo it). Take 100g of whipped cream to separate, store in the refrigerator to prepare the green tea mousse.
Heat the old milk, add white chocolate, turn off the heat, stir until the chocolate melts. Add the soft soaked gelatin, stir and dissolve (C).
Pour the mixture (C) into 150ml of whipped cream. Mix well to get the mixture (D).
Add the whipped whites (D). Such is the completion of white chocolate mousse. Pour the white chocolate mousse into the prepared cake mold. Spread the mousse evenly and put in the refrigerator for about 40 minutes to make it appear.

Prepare green tea mousse:
Soak gelatin in water. Whip the egg yolks and sugar until they turn white, smooth – called a mixture (E).
Mix green tea into hot water. Add stirring milk. Heat on low heat to melt tea. Add the soft soaked gelatin, stir the mixture (F).
Pour (E) into (F) and mix well (G).

Mix (G) with 100ml whipped cream and keep cool. Such is the completion of green tea mousse.
Gently pour the green tea mousse over the white chocolate mousse. Seamless flat cake surface. Refrigerate for at least 2 hours for the mousse to solidify.
Once the mousse has thickened, sift the green tea powder over the face and decorate as desired