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.
Notes
Ingredient Notes and Shopping Tips
Butter #1: For the cake portion, use softened butter. To quickly soften butter, cut it into smaller pieces or grate it using a cheese grater.
Milk: For best results, use whole milk. Low-fat or plant-based milks are not recommended.
Flour: Measure your flour by spooning it into your measuring cup and leveling it with a knife. Do not scoop your cup into your flour container, you may overmeasure.
Baking Powder: Replace every 6-12 months for the best rise.
Butter #2: For the filling and topping, be sure to use cold butter, not melted butter.
Tips and Tricks
Do not overmix: Mix your ingredients until the flour is just combined.
Pan Prep: Line your pan with nonstick foil or parchment paper to make removing the cake and cleanup a breeze.
Picture-Perfect Swirl Every Time: For the perfect cinnamon swirl, drop the streusel ingredients using a teaspoon from your silverware drawer. Then, swirl with a butter knife.
Don't Overbake: To tell if the cake is done, insert a toothpick in the center.