To make the topping:
Melt the butter in a medium saucepan over a low flame. Remove from the heat and cool for about 5 minutes, but do not allow the butter to become cold.
Add the flour, brown sugar, salt, vanilla extract, and cinnamon. Stir with a table fork until the mixture forms small crumbs. Set aside.
To make the cake:
Preheat the oven to 350 degrees with the rack in the lower third of the oven.
Butter a 9- by 13-inch pan. Dust the pan with all-purpose flour, then invert pan over the kitchen sink and tap to remove excess.
Combine the flour, baking powder, baking soda, and salt in a large bowl, then whisk together. Set aside.
Cut the butter into 1-inch pieces. Place them in the large bowl of a standup electric mixer fitted with beaters or a paddle attachment. Soften the butter on low speed. Increase the speed to medium-high and cream until smooth and light in color, about 1 1/2 to 2 minutes.
Add the sugar, 1 tablespoon at a time, taking about 6 to 8 minutes to blend it in well. Scrape the sides of the bowl occasionally.
Add the eggs, 1 at a time at 1-minute intervals, scraping the sides of the bowl as necessary. Beat for about 1 minute longer. Blend in the vanilla.
Reduce the mixer speed to low. Add the dry ingredients alternately with the sour cream, dividing the flour mixture into 3 parts and the sour cream into 2 parts, starting and ending with the flour. Mix just until incorporated after each addition. Scrape the sides of the bowl as necessary and mix for 10 seconds longer.
Turn the batter into the prepared pan, smoothing the surface with the back of a tablespoon or rubber spatula.
Take a handful of the crumb mixture and make a fist to press the mixture into a large clump. Then separate into smaller clusters, scattering them on the top of the cake batter. Repeat until all of the crumbs have been used. There should be enough to coat the cake all over.
Gently pat the crumbs into the batter with the palm of your hand; but do not press hard.
Bake in the preheated oven for 30 minutes, or until the cake is golden brown on top and begins to come away form the sides of the pan.