Preheat oven to 350 degrees F. Spray a 9x9 pan with non stick baking spray.
In a large bowl, cream butter and white sugar. Stir in vanilla, egg, baking powder, and salt.
Alternate stirring in flour and milk until just combined. Do not over mix.
Pour half of batter into prepared pan.
For the Cinnamon Swirl
In a small bowl, combine the butter, flour, brown sugar, and cinnamon.
Pour into the batter in pan, and swirl with a knife. Pour remaining batter on top.
For the Crumb Topping
In the same small bowl using 2 forks, butter, flour, brown sugar, and cinnamon until mixture resembles coarse crumbs.
Sprinkle topping on top of batter, and press in lightly.
Bake for 40-45 minutes, or until a tooth pick inserted in the center comes out clean.
Butter: Be sure to use softened butter, not melted butter.
Cinnamon: Make this recipe shine for any season by substituting apple pie, pumpkin, or gingerbread spice blends for the cinnamon.
Milk: For best results use whole milk.
Do not over mix: Mix your ingredients until the flour is just combined. Over-mixing can lead to a tough tense cake.
For the perfect cinnamon swirl drop streusel ingredients using a teaspoon from your silverware drawer. Then swirl with a butter knife.
To tell if the cake is done insert a toothpick in the center. If it comes out clean or with just a few crumbs, it is done. If it comes out with batter, continue baking in 2-3 minute increments. Do not over bake as that will lead to a dry cake.
Store in an airtight container for up to a week.
Freeze the whole cake or individual slices for up to 3 months.