Preheat oven to 350°F and grease two 8×4-inch loaf pans.
In a large bowl, whisk together the brown sugar and oil until no clumps of brown sugar remain. Whisk in the egg, followed by the buttermilk and vanilla. slowly whisk in the flour, baking soda, and salt, stirring until just combined.
To make the streusel topping, combine all the streusel ingredients into a small bowl and whisk until the mixture resembles coarse crumbs.
Pour a quarter of the batter into each of the loaf pans, using half the batter, total. Sprinkle half the streusel over the loaf pans, pressing down gently into the batter. Carefully spread the remaining batter over both loaf pans, topping with the remaining streusel.
Bake for 60 minutes, or until a tester inserted into the center of the bread comes out with a moist crumb.
Cool for 10 minutes in the pan, then turn out onto a wire rack and allow the bread to cool. Cut with a serrated knife.