For the Crumb Topping:

1. Preheat the oven to 350°F. Spray a 9×13 baking pan with cooking spray or line with parchment paper.
2. Melt the butter in a medium, microwave-safe bowl in the microwave in 30-second intervals. Stir in the brown sugar and sugar. Fold in the flour and cinnamon until the mixture is combined thoroughly and forms a dough. Set aside.

For the Cake:

1. In a separate medium bowl, whisk together the flour, baking powder, baking soda, and salt until combined. Set aside.
2. Cream the butter and sugar together in a large bowl with an electric mixer until light, fluffy, and well mixed. Mix in the eggs, one at a time, then add the vanilla extract and sour cream. Mix until combined.
3. Gradually mix in the dry ingredients. Continue mixing until the batter is thick. Spoon the batter into the prepared baking pan, and spread it out evenly.
4. Break the crumble topping apart into ½ inch pebble-sized pieces, and place them evenly over the cake batter so the whole cake is covered.
5. Bake on the middle rack for 45-50 minutes or until a toothpick inserted in the center comes out clean and the crumble has browned on top. If the crumble starts to look too brown at the 45-minute mark, cover the cake loosely with aluminum foil.
6. Let the cake cool to room temperature on a wire rack before serving. Dust with powdered sugar before serving (optional).