Latkes with Ancho-Chile Salt and Watercress Guacamole

Toast chiles in small skillet over medium heat until darker and aromatic, 1 to 2 minutes per side. Cut in half with scissors. Stem, seed, and tear chiles; grind finely with 1/2 teaspoon coarse salt in spice mill. Drape smooth kitchen towel over large bowl. Blend potatoes and 1 cup onions in processor until potatoes are very finely ground, scraping down bowl often. Scrape mixture into towel. Gather towel around tightly and squeeze out at least 1 cup liquid. Scrape dry potato mixture from towel into another large bowl. Add cilantro, egg, masa, 1/2 teaspoon pepper, 1 1/4 teaspoons coarse salt, and 1 cup onions. Stir until mixture becomes moist and sticks together. Heat 6 tablespoons oil in large nonstick skillet over medium-high heat. For each latke, drop 1 rounded tablespoonful potato mixture into skillet; flatten to 2 1/2-inch round. Fry latkes until golden brown, adding oil as needed, about 3 minutes per side. Transfer to rimmed baking sheet. DO AHEAD: Can be made 2 hours ahead. Let stand at room temperature. Rewarm in 400 °F oven until crisp, about 5 minutes per side. Arrange 3 latkes on each plate; sprinkle with chile salt. Spoon Watercress Guacamole into center. Garnish with watercress sprigs. Available at many supermarkets and at specialty foods stores and Latin markets.