01 - Preheat oven to 350°F. Grease and flour a 9-inch square baking pan.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl. Set aside.
03 - In a large bowl, beat softened butter and granulated sugar until light and fluffy, about 2 to 3 minutes.
04 - Add eggs one at a time, then mix in sour cream, milk, and vanilla extract until smooth.
05 - Gradually add dry ingredients to wet mixture, stirring just until combined.
06 - Mix brown sugar and ground cinnamon in a small bowl.
07 - Combine flour, brown sugar, cinnamon, and melted butter in another bowl. Stir until crumbly.
08 - Spread half of the batter evenly in the pan. Sprinkle cinnamon-sugar mixture over batter. Top with remaining batter and smooth surface.
09 - Sprinkle crumb topping evenly over the assembled layers.
10 - Bake for 38 to 42 minutes or until a toothpick inserted in center comes out clean.
11 - Let cool in pan for at least 15 minutes before slicing and serving.