Tip the brown sugar into a heatproof jug and pour on the boiling water and stir to dissolve.
Sieve the rice flour into a bowl and make a well in the centre. Mix in the sugar mixture and knead together to make a soft, but not sticky dough.
Dust your hands with rice flour and shape the dough into balls, roughly the size of golf balls.
Using your fingers, press an indentation in each dough ball to make a cup shape. From 1/3 cup chocolate, place a piece of dark chocolate inside each cup and press the edges of the dough around the chocolate so that the filling is completely covered.
Roll the balls in your hands so that they're all evenly shaped, and then coat each ball in sesame seeds.
Melt the remaining 1 1/2 cups/ 300 g chocolate in a double boiler or in a heatproof bowl placed over a pan of simmering water.
Heat the oil in a wok over a moderate heat (350 degrees F) and deep-fry the sesame seed balls, a few at a time, about 4, until the sesame seeds turn golden brown and the balls start floating to the surface, gently turning for the first 3 minutes, so they don't burn and left to float until golden. This should be around 4 to 10 minutes total.
Drain them on absorbent paper and arrange on a serving plate. Using a squeeze bottle or dipping a spoon, pour over the melted chocolate and serve warm.