"Slightly different from the other crumb cake recipes on the site in the butter and sour cream. From Bon Appetit."
dark brown sugar, packed
unsalted butter, melted
( still warm)
unsalted butter, room temperature
For the topping: Mix both sugars, cinnamon, and salt in medium bowl and whisk to blend. Add warm melted butter and stir to blend. Add flour and toss with fork until moist clumps form (topping mixture will look slightly wet). Set aside.
For the Cake: Position rack in center of oven and preheat to 350°F Butter 13x9x2-inch glass baking dish. Sift flour, baking soda, baking powder, and salt into medium bowl. Using electric mixer, beat room-temperature butter in large bowl 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; spread batter evenly with rubber spatula or 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 center comes out clean and topping is deep golden brown and slightly crisp, about 1 hour. Cool cake in dish on rack at least 30 minutes. DO AHEAD Can be made 1 day ahead. Cool completely. Cover and let stand at room temperature.
Cut cake into squares and serve slightly warm or at room temperature.