Preheat the oven to 350°F and grease an 8x8 or 9x9-inch baking pan.

Whisk the flour, granulated sugar, baking powder, baking soda, and salt in a medium bowl.

Whisk the milk, sour cream, melted butter, vanilla, and eggs in a second bowl until smooth.

Pour the wet mixture into the dry mixture and stir until the batter is fully combined.

Combine the flour, brown sugar, cinnamon, and salt for the crumb topping in a separate bowl.

Add the cold cubed butter and cut it in with a fork or your hands until chunky crumbs form.

Spread half of the cake batter into the prepared pan.

Sprinkle half of the crumb topping over the batter.

Spread the remaining batter on top.

Sprinkle the remaining crumb topping evenly over the surface.

Bake for 40 to 48 minutes, or until a toothpick inserted in the center comes out clean.

Cool for at least 15 minutes before slicing and serving.