Preheat the oven to 350°F (175°C). Grease a 9-inch square baking pan.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

In a large bowl, beat the butter and granulated sugar until light and fluffy, about 2 minutes. Beat in the egg and vanilla.

Add the dry ingredients to the butter mixture in three additions, alternating with the sour cream and milk, starting and ending with the dry. Stir just until combined.

For the filling, mix 1/4 cup brown sugar with 1 teaspoon cinnamon in a small bowl.

Spread half the batter in the prepared pan. Sprinkle the cinnamon sugar evenly over it, then scatter the chocolate chips on top. Dollop the remaining batter over the chips and spread gently.

For the crumb topping, combine 1/4 cup brown sugar, 1/3 cup flour, and 1/2 teaspoon cinnamon. Stir in the melted butter with a fork until clumpy. Sprinkle over the batter.

Bake for 35 to 40 minutes, until the top is golden and a toothpick inserted in the center comes out with moist crumbs. Cool in the pan on a wire rack for at least 15 minutes before slicing.