Mix the streusel: stir together the streusel flour, brown sugar, and cinnamon, then rub in the cold butter until it resembles wet sand. Set aside.

Whisk the 1 ½ cups flour, baking powder, baking soda, and salt in a medium bowl.

Beat the ½ cup butter with the ½ cup granulated sugar and ¼ cup brown sugar until light and fluffy, about 2 minutes. Beat in the eggs one at a time, then the vanilla.

Fold half the dry ingredients into the butter mixture, add the ½ cup sour cream, then fold in the remaining dry ingredients just until combined.

Preheat the oven to 350°F (175°C). Grease an 8 ½ x 4 ½‑inch loaf pan and line with parchment. Spread half the batter in the pan, top with the cinnamon‑sugar swirl mixture, then dollop and spread the remaining batter. Scatter the streusel over the top. Bake for 50 to 55 minutes, until a toothpick comes out with moist crumbs.

Cool in the pan for 10 minutes, lift out, and cool completely on a wire rack. Slice and serve.