Cinnamon Swirl Loaf Cake

0 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.
1 Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line it with parchment paper for easy removal.
2 In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
3 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.
4 Add eggs one at a time, beating well after each addition and scraping down the sides of the bowl as needed.
5 Mix in the sour cream, oil, and vanilla extract until fully combined.
6 Add the dry ingredients to the wet in two additions, mixing on low just until everything is incorporated. Do not overmix.
7 Spoon about half of the batter into a separate bowl and stir in the ground cinnamon until evenly mixed.
8 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!
9 Sprinkle the prepared streusel evenly over the top of the batter.
10 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.
11 Let the loaf cool in the pan for 10–15 minutes, then transfer it to a wire rack to cool completely.