Set oven to 350F (175C) with oven rack set to second-lowest position. Grease a 13 x 9-inch (or an 11 x 7-inch, baking time will need to be increased slightly). Cream butter with sugar (about 3-4 minutes), add in eggs and sour cream or crème fraiche; beat well (about another 3 minutes). In a bowl, sift together flour, baking powder and salt.Add the flour mixture to the creamed mixture; beat well until combined (batter will be thick). Spread more than half of the batter in the prepared baking pan. In another bowl, cream together all filling ingredients, then carefully spread over the cake batter. Spoon the remaining cake batter over the top of the cheese mixture. Mix all topping ingredients, pinching together with fingers until the consistency is like crumbs, and sprinkle over the top. Bake for about 50-55 minutes or until middle feels set and not jiggly.