Preheat the oven to 350°F or 175°C. Grease a 9 x 5-inch loaf pan and line it with parchment for easier removal.
Whisk flour, baking soda, cinnamon, and salt in a bowl. In another bowl, beat softened butter with brown sugar until creamy.
Add the eggs one at a time, mixing well after each addition. Stir in vanilla and sour cream until the mixture looks smooth and evenly blended.
Fold the dry ingredients into the wet mixture, stopping when a few flour streaks remain. Add the apples and fold just until the batter comes together.
Spread the batter into the loaf pan. Mix the streusel ingredients with your fingertips until crumbly, then scatter the mixture over the surface.
Bake for 50 to 60 minutes, until a tester inserted near the center comes out clean. Cool in the pan for 15 minutes before transferring the loaf to a rack.