How to Make It
Place chiles in a medium saucepan over medium, and cook, stirring occasionally, until lightly toasted, about 5 minutes. Stir in half-and-half, and bring to a simmer. Reduce heat to very low, and let mixture stand, undisturbed, 30 minutes. Pour chile-infused half-and-half through a wire-mesh strainer over a bowl; discard solids.
Place potatoes in a large saucepan, and add cold water to cover by 1 inch; bring to a boil over high. Add 1 tablespoon salt; reduce heat to medium, and simmer until tender, about 25 minutes. Transfer potatoes to a colander to drain, shaking off any excess water.
Stir together butter and chile-infused half-and-half in a saucepan over medium until butter is melted. Remove from heat. Press potatoes through a potato ricer into saucepan; stir mixture until combined. Season with remaining 4 teaspoons salt, and serve.
Cilantro Oil Drizzle: Puree 2 cups loosely packed fresh cilantro leaves, 1/2 cup canola oil, 1/2 cup extra-virgin olive oil, and 2 ice cubes in a blender until smooth. Season with salt to taste. Drizzle some oil on mashed potatoes just before serving; pass the remaining oil at the table.