Preheat oven to 350F/175C. Coat a 9x13-inch baking dish (the best approximation outside the US is a 3L rectangular baking dish) with non-stick spray.
For the cake:
Mix the flour, baking soda and baking powder in a small bowl; set aside.
Beat the sugar, butter and egg until smooth. Add the sour cream and vanilla; mix well. Stir in the 3 cups of flour mixture until a sticky dough forms. Spread 1/2 of the dough evenly in the prepared pan. TIP--I distribute portions evenly around the pan them pat it to fit with wet hands. Alternately, you could use an offset spatula, but the damp hands work better.
Beat the cream cheese, 1/2 cup sugar and egg in another bowl until very smooth. Spoon mixture over the dough in the pan and spread it as evenly as you can.
Drop the remaining dough by spoonfuls over the cream cheese mixture.
Mix the topping ingredients together. If the crumbs seem a bit wet, add a tablespoon or two more of flour. Sprinkle the topping over the crumbs.
Bake for 45 minutes, or until a toothpick inserted in the center comes out clean.