1. Soak noodles in water for 30 minutes or according to package directions. Drain; set aside.
2. In a small bowl, stir together brown sugar, fish sauce, vinegar and cayenne; set aside. Heat oil in a large nonstick skillet on medium-high heat. Add garlic; saute 30 seconds. Stir in shrimp; saute 2 minutes, flipping once. Move garlic and shrimp to side of pan closest to handle. Tilt skillet so that empty side is closest to burner; add noodles and 1/2 cup water. Stir noodles for 3 to 4 minutes, until almost all the water is absorbed, keeping skillet tilted and shrimp untouched. Return entire pan to burner. Pour in fish sauce-brown sugar mixture and stir, combining shrimp and noodles. Make a well in the center. Add 1 egg; when white starts to set (it takes about 1 minute), break yolk with a rubber spatula, scramble and mix into shrimp-noodle mixture. Repeat with remaining 2 eggs, making a well each time. Stir in bean sprouts, 3/4 cup of the scallions and the peanuts.
3. Serve immediately. Garnish with remaining 1/4 cup scallions and, if desired, chopped peanuts, a squirt of lime and fresh cilantro.