The first step is to boil the sweet potatoes to just the right consistency. If you over boil, then it does not bind well. Just when the potatoes have become soft, take them out and put them in cold water to stop the cooking process.
Then peel the skins and make a mash when completely cool. Add rice flour and all purpose flour to bind the sweet potato. In case, required, add a tbsp of superfine sugar to this.
The dough is a sticky dough and the rice flour helps to bind it well. If it is too sticky, then add a tsp more of rice flour. But adding too much rice flour will make it a hard dough and it will not taste good.
While the potatoes are boiling, also put the milk in a deep bottomed sauce pan for thickening. It takes time and patience to thicken milk- almost an hour or more. Keep stirring from time to time over low to meium flame and thicken the milk.
Alongside, the dumplings can be shaped. Make small balls of the sweet potato dough and take one ball at a time. Gently flatten on the palm of your hand and spoon in half a tsp of the coconut filling in it. Cover the filling with both ends of the dough and press with your finger to seal the edges. Then shape it in a cylinder with pointed edges. Arrange them on a dry plate and keep aside.
When the milk has reduced to 2/3 rd, add a tbsp of sugar and thicken it further.
Take 1/2 cup of the warm milk and add grated gur to it. Mix it and make a slurry and keep aside.
When the milk has thickened, add the dumplings one by one and let the milk boil. When it starts to bubble on the sides, add the gur slurry, give it a gentle stir and turn off the heat.
You can serve ranga alur dudh puli only when it has cooled down and it tastes better when chilled. Also, it takes a few hours for the flavours to infuse. This can be stored in the refrigerator for 4 - 5 days at the most.