Spray inside of 6-quart slow cooker with no stick cooking spray. For the Chocolate Cake, prepare cake batter as directed on package, stirring in cinnamon and vanilla. Pour into greased slow cooker. Place towel over slow cooker and cover with lid.
Cook 2 hours on LOW or until cake is almost set.
For the Pecan Pie Topping, sprinkle pecans over cake. Beat remaining ingredients with wire whisk until smooth. Slowly pour over pecans. Cover.
Cook 10 minutes longer on LOW. Turn off slow cooker. Carefully remove slow cooker insert and place on wire rack. Cool 20 minutes before serving. Serve with vanilla ice cream, if desired.