1. Preheat oven to 350°F (176°C) and grease a 9×13 inch cake pan.
2. To make the cinnamon filling, combine the brown sugar and cinnamon in a medium sized bowl.
3. Add the melted butter and mix until combined. Set aside.
4. To make the cake batter, combine the sugar and butter to a large mixer bowl and beat on medium speed until light in color and fluffy, 3-4 minutes. Don’t skimp on the creaming/beating time.
5. Add the sour cream and mix until well incorporated.
6. Add the milk and mix until well incorporated.
7. Add the eggs and vanilla extract and mix until smooth.
8. Combine the dry ingredients in a separate bowl.
9. Add the dry ingredients to the batter and mix until smooth.
10. Pour the batter into the prepared pan and spread into an even layer.
11. Drizzle the cinnamon sugar mixture over the top of the cake. It will cover most of the top of the cake.
12. Use a toothpick to swirl the cinnamon mixture around. You may not notice a difference immediately, but the swirls are where the cinnamon mixture will fall down into the cake while baking.
13. Bake the cake for 22-24 minutes, or until a toothpick inserted comes out with a few crumbs, then remove from the oven to cool.
14. To make the icing, combine all the icing ingredients except for the powdered sugar in a small sauce pan.
15. Heat on low heat until the sugar is melted, then turn heat to medium to bring to a boil.
16. Boil for 3 minutes, stirring occasionally, then remove from heat and immediately pour into a heat-proof glass bowl.
17. Whisk in the powdered sugar and allow to cool slightly and thicken. If you find your icing to be a little thick, just add a touch more water to thin it back out. If it’s too thin, add a little more powdered sugar.
18. Pour the icing evenly over the cake. Serve warm. If some cake is left over, store covered in the fridge. Warm back up to serve leftovers.