Before you start the sauce, first prepare your mold which can be a cake pan, a casserole dish, or any kind of containers to shape the noodles. This recipe fits a 9-inch square pan perfectly, but you can use multiple containers or even half-fill a bigger pan. Line the bottom of the pan with parchment or foil to help it come out more easily, though this is not necessary as you can cut them right in the pan.
In a large non-reactive wok or skillet, saute the shallots in the oil over medium low heat until soft and caramelized, 5-6 minutes.*Ideally you want to use a large non-reactive wok (so not carbon steel or cast iron) so you can mix the noodles right in there. If you don't have it, you can make the sauce in a stainless steel skillet, and then toss it with the noodles in a large mixing bowl.
Once the shallots are done, add the egg and scramble into tiny pieces.
Add the palm sugar, fish sauce, tamarind, sriracha, lime juice and lime zest; keep cooking on medium low heat, stirring constantly, until it reaches a temperature of 240°F/115°C. I do not recommend you eyeball this; use a thermometer to be sure otherwise your noodles might not stay crispy after tossing with the sauce. See post above for more details.
Once the sauce is ready, turn the heat down to the lowest heat and add all of the noodles to the wok. Crush the noodles down so that they will fit into the wok, then use 2 spatulas to quickly but thoroughly toss the noodles with the sauce until they are evenly coated. If the pan you're using for the sauce is not big enough, drizzle the sauce over the noodles in a large mixing bowl, then toss.*IMPORTANT: Keep an eye out for any clumps of sauce and break it apart. These clumps will end up being overpowering bits that are too sweet, and it will take away sauce from the other parts of the noodles. This is another reason why ideally you want to toss noodles in the wok, so we can keep the heat on for a bit to keep the sauce from stiffening as it cools, making it easier to mix.
While still warm, put half of the noodles into the parchment-lined square cake pan or casserole dish and spread it out to fill the pan evenly. Sprinkle half of the cashews and pumpkin seeds over, if using, then spread the remaining noodles on top, followed by the remaining cashews and pumpkin seeds. Then use a measuring cup with a flat bottom to push the noodles down to pack them so they will hold together when cut, but without crushing the noodles.
Let cool completely, then either cut it in the pan or unmold it and cut it on a cutting board. Keep the pieces in an airtight container along with fried lime leaves and chilies, if desired. You can enjoy them now, but if possible wait at least half a day before serving to allow the flavours to settle into the noodles. You can eat the fried chilies if you like it spicy, but do not eat the dried lime leaves, they're just for aroma and garnish!
Storage: They will keep for at least a week in an airtight container at room temperature. Put them in the fridge if you're in a humid climate. See FAQ above for more details.