Mix the first four ingredients together( by hand, about 50 strokes). It will still have some small lumps. Pour half the batter into an ungreased 9 x 13" glass baking dish. Combine the brown sugar and cinnamon, sprinkle over the batter in the cake pan. Spoon the other half of the batter into the cake pan, covering the brown sugar and cinnamon. Twirl the cake with a butter knife or icing knife into whatever design you want to make.
Bake for 40 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Mix the confectioners sugar, milk and vanilla in a bowl with a whisk. Drizzle over warm cake and enjoy.