Combine urad dal and rice in 3~4 cups of water. Soak for 4~6 hours. Drain and grind to a smooth batter using enough water. Batter should have the consistency of heavy cream. Set aside.
Prepare the Filling:
Combine chana dal with water and cook in the pressure cooker until tender but not really mushy. Drain well and reserve the cooking water to make dal or sambar.
Cool the cooked dal and mash it into a paste.
In a medium size heavy bottom pan on medium heat, cook jaggery until melted. Add the mashed lentils and cook till the mixture comes together and is not sticky anymore. Add coconut and cook for 1 more minute. Turn off the heat and let the mixture cool for 10 minutes. Then divide the filling into equal size balls. Place them on a plate to cool completely.
Heat oil in a medium size heavy bottomed wok or pan. Test the oil by gently putting a drop of batter and if it sizzles and comes to the surface then it is ready for frying.
Take one filling ball and dip it in the covering batter. Roll it to cover evenly with the batter. Remove and gently slide it into the hot oil. Cook, stirring occasionally, until boorelu are golden brown on all sides. Depending on the size of your pan, you can make multiple boorelu at one time. Once done, remove with a slotted spoon onto a paper towel lined plate. Repeat with remaining filling balls.
Serve warm or at room temperature.
The first and foremost if to make sure that the stuffing is not too wet or not too dry. When you are melting jaggery, the mixture tends to get watery. So make sure to cook it in a nonstick pan until the mixture thickens and dries up. This usually takes about 5~8 minutes on medium-heat cooking.
Make sure that the batter for outer cover is not too thick or thin. It should have a thick heavy cream consistency.
You can make the batter for the outer cover using store bought flours to save soaking time. Use ¼ cup urad flour and ¾ cup rice flour. Mix well, add enough water to make a thick yet pourable batter. Cover and set aside for about 1~2 hours. Use according to the recipe.
The sweet filling can be made up to 3 days and stored in an airtight container in the fridge.
Batter for the outer cover can also be made 2 days ahead of time and stored in the refrigerator.
Make sure that the oil is well heated before adding the boorelu for frying. Do not crowd the pan.
The trick is to make sure that the stuffing is not too wet or not too dry. It generally tends to get watery after adding jaggery. Then, cook it in a nonstick pan until the mixture dries up. This usually takes about 5~8 minutes.
To test the oil by gently putting a drop of batter and if it sizzles and comes to the surface then it is ready for frying. Otherwise, heat the oil for few more minutes.
Leftover burelu can be stored in an airtight container for 1~2 days at room temperature. These do not need to be refrigerated. They might not be as crispy the next day.