For the crumbs: mix the sugars, cinnamon and salt in a medium bowl. Add the warm melted butter and stir to blend. Add the flour and mix until crumbs begin to form (it will clump). Set aside.
For the cake: Preheat oven to 350F/180C and grease a 9 x 13-inch glass baking dish.
Sift together the flour, baking soda, baking powder and salt in a medium bowl; set aside.
Beat the butter in the bowl of a stand mixer--the batter is dense so it will require a sturdy mixer--until smooth. Add the sugar and beat until it's light and fluffy.
Add the eggs, beating well after each addition. Scrape down the sides and bottom of the bowl.
Add the sour cream and vanilla and mix until just incorporated. Don't overmix!
Add the flour in 3 additions, beating until just incorporated. Again, do not overmix.
Pour batter into prepared pan and spread evenly in the pan.
Squeeze small handfuls of the crumbs together to form small clumps. Drop the clumps evenly over the batter, covering the top completely. The crumb layer will be thick.
Bake for 50-60 minutes or until a toothpick inserted in the center comes out clean. Avoid overbaking.
Cool at least 30 minutes on a rack before serving.