Preheat oven to 350°F. Spray a 9x5-inch loaf pan with nonstick spray. Line the pan with parchment paper, leaving an overhang on the sides, and spray with more nonstick spray for easier removal. Set aside.
In a small bowl, mix the brown sugar and cinnamon together with a fork. Set aside.
⅔ cup brown sugar, 1 tablespoon ground cinnamon
In a medium bowl, whisk the flour, baking powder, salt, and sugar together. Set aside.
2 cups all-purpose flour, 1 tablespoon baking powder, ½ teaspoon kosher salt, ¾ cup granulated sugar
In a large bowl, whisk the egg, milk, and vegetable oil together until combined. Pour the flour mixture into the milk mixture and fold it together with a spatula just until combined. Don’t overmix.
1 large egg, 1 cup milk, ⅓ cup vegetable oil
Pour half of the batter into the prepared pan, and sprinkle half of the brown sugar and cinnamon mixture over that layer. Pour the remaining batter on top, spreading to the edges, and then sprinkle the remaining brown sugar and cinnamon mixture all over the top.
Bake for 45-55 minutes, until a toothpick inserted comes out clean. Cool in the pan for 10 minutes, then lift it out of the pan and finish cooling on a rack.