Preheat oven to 350 degrees. Prepare a 9 inch cake pan with a circle of parchment paper in the bottom and grease the sides.
To prepare the streusel, add flour, brown sugar, cinnamon, melted butter and salt to a medium sized bowl and mix with a fork until all ingredients are incorporated and crumbly. Set aside.
To make the cake batter, combine the flour, baking powder and salt in a medium sized bowl and set aside.
Add the sugar and butter to a large mixer bowl and beat together until light in color and fluffy, about 3-4 minutes. Do not skimp on the creaming time.
Add sour cream and mix just until well combined.
Add the milk and mix just until well combined.
Add egg and vanilla extract and mix just until well combined.
Add the dry ingredients to batter and mix until well combined.
Spread half of the cake batter into the bottom of the cake pan. The batter will be thick and the layer will be thin.
