In a large pot boil the water, rice, salt and bouillon. Cook on medium high, until the rice is done.
Meanwhile in a bowl, mix yogurt, milk, egg yolk and flour. After the rice is cooked, take a few spoons of liquid from the pot and mix into the bowl. Then slowly pour your mixture into the pot while stirring very slowly. Cook for 10 more minutes on low heat.
Place the butter in a frying pan. When it begins spitting, stir in the mint and pour into your soup.