Preheat oven to 350°F. Using an electric mixer, beat cake ingredients together for 2 minutes. Scrape the bowl halfway through. Pour into a greased 9 x 13 pan.
Combine filling ingredients and sprinkle over batter. Take a butter knife and swirl filling and batter together. Swirl until clumps of brown sugar have sunken down into the batter.
Mix topping ingredients together and sprinkle over the top of the batter.
Bake for 35 - 45 minutes or until a cake tester inserted in the center comes out clean.
While cake is baking, combine icing ingredients in a bowl and whisk until smooth. Pour over cake immediately after it comes out of the oven. While the cake is still hot cover with foil and let cool.