01 -
Preheat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line it with parchment, leaving overhang on two sides.
02 -
In a large bowl, whisk the flour, granulated sugar, baking powder, baking soda, salt, and cinnamon until evenly combined.
03 -
In a second bowl, whisk the buttermilk, oil, eggs, and vanilla until smooth.
04 -
Pour the wet ingredients into the dry ingredients. Fold with a spatula just until the flour is moistened and no dry streaks remain.
05 -
Stir the granulated sugar, brown sugar, cinnamon, and melted butter in a small bowl until crumbly and evenly damp.
06 -
Spread half the batter in the pan. Sprinkle with half the cinnamon mixture. Add the remaining batter and the remaining cinnamon mixture. Drag a knife through the batter 5 or 6 times to create a loose swirl.
07 -
Bake for 50-60 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs. Tent loosely with foil if the top browns early.
08 -
Cool in the pan for 12 minutes, then lift onto a wire rack. Whisk powdered sugar, milk, and vanilla until smooth. Drizzle over the barely warm loaf before slicing.