Preheat the oven to 375ºF. Grease a 10x15x1-inch baking pan.

In a saucepan, combine the butter and the water and bring to a boil. Meanwhile, in a large bowl, combine the flour and sugar. Add in the eggs, sour cream, vanilla, baking soda and salt. Mix to combine.

Pour the boiling water/butter mixture into the flour mixture, a little at a time, stirring constantly. The batter will be very thin. Pour the batter into the prepared pan.

In a small bowl, combine the brown sugar, butter, cinnamon and flour. Place dollops of the mixture over the cake batter, distributing it evenly. Use a knife to gently swirl the filling, making sure not to mix.

Bake the cake for 18-22 minutes, or until golden brown. Remove from the oven and let the cake cool at least 5-10 minutes before frosting.

To make the frosting, beat together the cream cheese and butter until fluffy. Add in the powdered sugar a little at a time, then add the vanilla. Beat for 2-3 minutes. Spread onto the cake. Cut the cake into squares to serve.