- ½ cup all-purpose flour
- ¼ cup packed golden brown sugar
- ¾ cup granulated sugar
- 4 tablespoons cold butter (½ stick), cut into small pieces
- 1 cup chopped pecans, walnuts or a mixture
- 1 tablespoon ground cinnamon
- 1¼ cups granulated sugar
- 1½ sticks unsalted butter, softened
- 3 large eggs
- 1 teaspoon pure vanilla extract
- ¼ cup buttermilk
- 1 cup sour cream
- Zest of 1 small orange or tangerine
- 2½ cups cake flour
- 1½ teaspoons baking powder
- ½ teaspoon baking soda
- ½ teaspoon sea salt (reduce this amount if using salted butter)
- ½ cup peeled, diced apple
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.