In a small bowl, combine the flour, brown sugar, and cinnamon. Add the softened butter and use a fork or your fingers to mix until the topping resembles coarse crumbs with no dry flour left. Set aside.
Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line it with parchment paper for easy removal.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large bowl using a hand mixer (or a stand mixer with the paddle), beat the butter and sugar together on medium-high speed until light and fluffy, about 2–3 minutes.
Add eggs one at a time, beating well after each addition and scraping down the sides of the bowl as needed.
Mix in the sour cream, oil, and vanilla extract until fully combined.
Add the dry ingredients to the wet in two additions, mixing on low just until everything is incorporated. Do not overmix.
Spoon about half of the batter into a separate bowl and stir in the ground cinnamon until evenly mixed.
Alternate spoonfuls of the plain and cinnamon batters into the prepared loaf pan. Use a butter knife to gently swirl them together — don’t overdo it!
Sprinkle the prepared streusel evenly over the top of the batter.
Bake for 55–65 minutes, or until a toothpick inserted in the center comes out clean. Check at the 50-minute mark and tent with foil if the top is browning too quickly.
Let the loaf cool in the pan for 10–15 minutes, then transfer it to a wire rack to cool completely.
