Scald the cream and pour over the chocolate. Stir occasionally until incorporated. If chocolate is not completely melted, heat gently over a hot water bath and stir until incorporated. Let cool at room temperature overnight. Transfer to a mixer attached with a paddle. At medium speed whip the chocolate and sesame paste until softened well. Using a little ice cream scoop, make small balls and refrigerate for 30 minutes. When chilled roll in toasted sesame seeds. (These are the truffles) Take half of the balls and wrap in the rice dough. In a pot of boiling water, poach balls for 3 minutes. Serve in bowl of broth.
For the rice dough: In a bowl, combine the starches and slowly pour in the water, mixing constantly with a wooden pin. Add the oil and mix until well incorporated. (You can use a mixer with a dough hook.) Roll into a log (3/4-inch diameter) and cut into 1/2-inch pieces. Roll out into small circles, 1/8-inch thick.
For the soup: In a saucepan, heat water, sugar and lemon grass until a simmer. Reduce by 50 percent. Let steep for 30 minutes. Strain the liquid and bring to a boil. Whisk into the boiling soup to thicken slightly.
Plating: In small soup plates, place 3 rice balls in some hot broth. For the truffles, serve on a plate garnished with cocoa powder.