Bring a pot of water large enough to cook the rice noodles to a boil
Wisk the stock and coconut milk together and bring to a simmer. Add the chicken breasts (or thighs) to the liquid and simmer gently until they reach 160F for white or 170F for dark. Set aside to rest.
Strain the liquid if desired and return to the pot. Your call. The broth will have little bits in it. If that bothers you then strain. If you don't care then don't.
Return the broth to a simmer. Add the red Thai curry and tom yum pastes into a bowl. Ladle in a little of the coconut milk broth and mix to combine. Trust me - it's worth the extra dishes so you don't have to chase bits of curry paste around in the pot.
Add the red curry/tom yum mixture back into the coconut milk broth along with 2 tsp of brown or palm sugar. Taste. If you like things a little sweeter add another tsp of sugar. If you want things saltier add a bit of fish sauce. You probably won't need it.
Creep up on both the salt and sugar. Adding more is easy. Taking it away is impossible.
Slice or shred your chicken. Sliced if you want things pretty. Shredded if you like bigger bites. Sliced makes for nicer pictures but I like shredded better.
Add the rice noodles to the boiling water. Check after 30 seconds. If they are still toothy let them go another 30 seconds and check again. You want a bit of chew to the noodles.
When you are happy with your noodles, drain and divide into two large bowls.
If you sliced and are going for pretty then fan the chicken over the noodles and pour the broth overtop. If you shredded then the chicken can go in when you like.
Garnish with your choice of sliced shallots, sliced green onion and lime. Enjoy!