1/3 cup granulated sugar
1 tablespoon cinnamon
1/4 cup unsalted butter
1 large egg
1 cup packed light brown sugar
1/4 cup canola (or vegetable) oil
1 tablespoon vanilla extract
1 cup buttermilk
2 cups all-purpose flour
3/4 teaspoon baking soda
Preheat the oven to 350°F. Spray a 9-by-5-inch loaf pan with cooking spray, then dust with flour. Set aside.
In a small bowl, stir together the sugar and cinnamon. Set aside.
In a large microwave-safe bowl, microwave the butter until melted, about one minute. After the butter has cooled slightly, add the egg, brown sugar, and vanilla. Whisk to combine. Add the buttermilk and whisk to incorporate. Add the flour, baking soda, and salt, and stir until just combined. The batter will be lumpy. Do not overmix.
Pour half of the batter into the prepared pan, then smooth with a spatula. Sprinkle approximately three-quarters of the cinnamon-sugar mixture evenly over the surface of the batter. Top with the remaining batter, and smooth with a spatula. Sprinkle the remaining cinnamon-sugar mixture evenly over the top.
Bake for 45 to 55 minutes, or until the top is set and firm, and a toothpick inserted into the center comes out clean or with only a few moist crumbs. Cool in the pan for 15 minutes, then invert and let cool completely.