Butter a 10-inch tube pan or 9- by 13-inch cake pan. Preheat oven to 350°.
Combine the streusel topping ingredients in a bowl and cut together with a pastry blender, fork or fingers until the mixture resembles coarse crumbs. Set aside.
Cream sugar and butter with the paddle attachment of an electric mixer until light and fluffy, about 3 minutes (or by hand with a wooden spoon if you’re feeling strong). Stir in the eggs 1 at a time until blended and then add the vanilla, buttermilk, sour cream and citrus zest.
Sift together the flour, baking powder, baking soda and salt onto a piece of parchment paper and then fold the dry mixture into the batter until just combined.
Spoon half of the batter into the prepared pan; sprinkle with the apples, half of the streusel topping mixture and swirl with a spoon. Add the remaining batter and then top with the remaining streusel topping.
Bake 45 minutes to an hour, or until a skewer inserted in the center comes out clean.