Prep oven and pans. Position rack to center of oven and preheat oven to 350°F (180°C). Grease a 9×9 (or 8×8) pan with butter or non-stick cooking spray.

Make the topping. Whisk brown sugar, cinnamon, and salt in medium bowl until evenly combined. Add warm melted butter, and with a wooden spoon or spatula, stir to blend. Add flour and toss with fork until moist clumps form (topping mixture will look slightly wet). Stir in the chocolate chips. Set aside.

Combine dry ingredients. In a small mixing bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.

Cream butter and sugars. In a large mixing bowl, with an electric mixer cream butter and sugars together until light and fluffy, about 2 minutes.

Add wet ingredients. Add eggs, one at a time, mixing until incorporated. Scrape down the bowl as needed. Add vanilla and mix to combine.

Alternate dry with sour cream. Add the flour mixture, alternately with the sour cream in 3 additions, starting and ending with the flour.

Spread half of the batter in pan. Top with half of the brown sugar topping. Add remaining batter. Add remaining streusel.

Bake. Bake for 30-40 minutes or until golden brown, and the inserted toothpick comes out clean. Remove from the oven and let cool before serving.