Preheat oven to 300 degrees. Grease a 9 x 13 inch dish. In a mixing bowl, combine cake ingredients. Pour half into prepared dish. Combine sugar, cinnamon and raisins, if using. Sprinkle over batter. Swirl through with knife. Bake for about 45 minutes to 1 hour.
In a small bowl, sift powdered sugar. Whisk in milk, 1 tablespoon at time to make a thick or thin as you'd like. Add vanilla. Pour over cake as soon as you take it out of the oven. Serve warm.