Clean, wash and cut the rhubarb into pieces. Mix groats powder with sugar. Gradually stir with 125 ml water until smooth. Bring 900 ml water to the boil, remove from the heat and stir in the mixed powder. Add rhubarb and bring to the boil. Simmer over medium heat for about 5 minutes. Remove from the heat and let it cool down, stirring occasionally.
Peel bananas and cut into pieces. Add lemon juice and purée. Put milk into a bowl and stir in the cream powder with the whisk of the hand mixer first at a small, then 3 minutes at the highest setting. Stir in the banana puree. Dip the edges of the glasses first in water, then in pink sugar. Pour the groats and banana cream into the glasses and serve immediately