Preheat oven to 350°F. For the Topping, mix flour, sugar and cinnamon in medium bowl. Add melted butter; toss with spatula until large crumbs form. Stir in nuts. Set aside.
For the Cake, beat cake mix, egg, sour cream, butter and vanilla in large bowl with electric mixer on medium speed about 1 minute or just until mixed.
Spread evenly in greased and floured 13x9-inch baking pan. Sprinkle with topping mixture.
Bake 25 to 30 minutes or until cake pulls away from sides of pan. Cool on wire rack.