Preheat your oven to 350°F (175°C) and grease a 9×13 inch baking dish.
In a large bowl, mix the flour, sugar, baking powder, baking soda, and salt.
In a separate bowl, whisk together the sour cream, egg, vanilla extract, and melted butter until well combined.
Combine the wet ingredients with the dry ingredients and stir until just mixed.
Pour the cake batter into the prepared baking dish and spread evenly.
In another bowl, combine the brown sugar and cinnamon.
Add the chilled cubed butter to the sugar and cinnamon mixture, using a pastry cutter or fingers to create a crumbly texture.
Sprinkle the crumb topping evenly over the cake batter.
Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out clean.
Allow the cake to cool completely in the pan before cutting into bars.
Variations & Tips
For a nutty twist, consider adding 1/2 cup of chopped walnuts or pecans to the crumb topping. You could also introduce a hint of almond flavor by substituting half of the vanilla extract with almond extract. If you’re feeling adventurous, try incorporating fresh blueberries or raspberries into the batter before adding the crumb topping for a fruity burst of flavor. For those seeking a slightly less sweet version, reduce the granulated sugar in the batter to 3/4 cup.
