Prep oven and pan. Position rack to center of oven and preheat oven to 350°F (180°C). Grease a 9x5 loaf pan with butter or non-stick cooking spray.
Make topping. In a small mixing bowl, stir together brown sugar, cinnamon, and flour. Cut in the butter using a pastry cutter or rub in using your fingertips. Set aside. ⅓ cup (67 g) packed light brown sugar 6 Tablespoons (45 g) all purpose flour 1 teaspoon ground cinnamon 3 Tablespoons (21 g) unsalted butter, softened
Combine dry ingredients. In a small mixing bowl, whisk together flour, baking powder, baking soda, and salt. Set aside. 1 cups (120 g) all-purpose flour ½ teaspoon baking powder ⅛ teaspoon baking soda ¼ teaspoon salt
Cream butter and sugars. In a large mixing bowl, with an electric mixer cream butter and sugars together until light and fluffy, about 2 minutes. ¼ cup (½ stick, 57 g) unsalted butter ¼ cup (50 g) granulated sugar ¼ cup (53 g) packed light brown sugar
Add wet ingredients. Add the egg and mix until incorporated. Scrape down the bowl as needed. Add vanilla and mix to combine. 1 large egg 1 teaspoon vanilla extract
Alternate dry with sour cream. Add the flour mixture, alternately with the sour cream in 3 additions, starting and ending with the flour. ½ cup (113 g) sour cream
Assemble apples. In a small bowl, stir together the apples and cinnamon. 1 ½ cups peeled and diced apples ¼ teaspoon ground cinnamon
Assemble cake. Spread half of the batter in the pan. Layer on half of the apples in an even layer. Crumble on half of the streusel topping. Repeat with remaining batter, apples and streusel.
Bake. Bake for 40-45 minutes or until golden brown, and the inserted toothpick comes out clean. Remove from the oven and let cool.
Make icing. In a small bowl whisk together the powdered sugar, vanilla, milk and cinnamon. Drizzle over the cooled cake. ½ cup (57 g) powdered sugar ½ teaspoon vanilla extract ½ teaspoon vanilla extract ¼ teaspoon ground cinnamon