Position rack in centre of oven and preheat to 350°F.
Butter 13 x 9 x 2-inch baking dish or baking pan.
Sift flour, baking soda, baking powder and salt into medium bowl.
Using an electric mixer, beat room temperature butter until smooth.
Add sugar and beat until light and fluffy.
Add eggs 1 at a time, beating until well blended after each addition.
Add sour cream and vanilla extract and beat just until blended.
Add flour mixture in 3 additions, beating just until incorporated after each addition.
Transfer cake batter to prepared baking dish and spread batter evenly with an offset spatula.
Squeeze small handfuls of topping together to form small clumps.
Drop topping clumps evenly over cake batter, covering completely (topping will be thick).
Bake cake until tester inserted into centre comes out clean and topping is deep golden brown and slightly crisp, about 1 hour.
Cool cake at least 30 minutes.
Cut into squares and serve slightly warm or at room temperature.
Recipe from Epicurious