In a pestle and mortar add 5 cloves of garlic, 1/4cup roughly diced coriander/cilantro roots & stems, 1 tsp sugar and 1/2 tsp salt, white & black pepper. Grind into a smooth paste.
Scoop into a large mixing bowl and stir in 1 tbsp soy sauce and 2 tsp oyster sauce. Add in chicken pieces and coat with your hands. Really rub the marinade into the meat. Pop cling film over the top and place in the fridge. I recommend a minimum of 4 hours, but marinate for as long as you have time for (best results over night).
Meanwhile, place your diced shallots between two paper towels. Firmly press down with your hands to squeeze out as much moisture as possible. Place to one side.
Heat up oil in a large pot to 120C/250F and add in your shallots. Make sure the heat does not creep any higher, you want to fry them fairly low and slow to bring out as much moisture as possible, so they turn out extra crispy. Fry until they start turning from a light golden colour, to a slightly deeper brown. These go from perfect to burnt VERY quickly. Once cooked remove and lay on paper towels.
Heat up oil to 175C/350F. In a medium sized mixing bowl mix together tempura batter and chicken powder. Pour in COLD soda water and stir until the lumps smooth out. You want the consistency like heavy/double cream. Take a piece of chicken and coat in the batter, then carefully lower into the oil. You'll need to do this in batches of 3-4. I usually pop an ice cube in the batter to keep it cold. The colder it is the crispier the chicken will be.
The temp will drop to around 165C/330F, and that's fine, just keep it on that heat. Depending on the size of the chicken pieces it'll take around 5-8mins on each side. Once cooked rest on cooling rack with paper towels underneath. You want the chicken to be brown and crispy on the outside, with the centre piping hot (internal temp 165F/75C - take into consideration it continues cooking slightly once out the oil). See notes if at all weary.
Season the chicken with a final pinch of salt (optional) and serve with crispy shallots on top with sweet chilli sauce on the side. Enjoy!