Preheat oven to 350°F.
In a medium mixing bowl, combine all cake ingredients. Batter will be thick. Pour into a greased 8x8 baking dish.
Combine ingredients for the cinnamon filling. Drop spoonfuls over batter, and swirl into cake batter using a toothpick or skewer.
Bake for 35 minutes or until a toothpick inserted into the center of the cake comes out clean.
Meanwhile, beat icing ingredients over low speed until no lumps remain.
Allow cake to cool completely before adding icing. Ice using a spatula or piping bag.