Bake cake according to package instructions. Cool completely.
Place chocolate in a microwave-safe bowl. Microwave at HIGH for 30 seconds or until melted, stirring every 10 seconds. Drizzle chocolate over cake. Combine brown sugar and next 3 ingredients (through salt) in a small, heavy saucepan over medium-high heat; bring to a boil, stirring just until sugar dissolves. Reduce heat, and cook 4 minutes (do not stir). Remove from heat; quickly drizzle caramel over cake. Top with pecans.