- Melt the chocolate. Fill a large (4-quart) saucepan half full of water and bring to a simmer over medium. Scoop both kinds of chocolate into a metal mixing bowl (preferably one that fits your mixer), set the bowl over the pan of simmering water and stir until chocolate melts. Scrape the melted chocolate in a mixer bowl if it isn’t in one.
- Make the frosting. Sift the sugar over the melted chocolate, drizzle in ¼ cup warm tap water and mix on low speed until combined. Scoop in the sour cream in three additions, mixing well after each addition. Sprinkle in the vanilla and both chile powders. Add the butter several cubes at a time, mixing well after each addition. Increase speed to medium to beat the mixture until the butter is incorporated and the frosting is smooth and shiny. Chill for at least one hour before decorating your cake.