Preheat oven to 350F. Spray a 9-by-5-inch loaf pan with floured cooking spray, or grease and flour the pan; set aside.

In a small bowl, combine 1/3 cup granulated sugar with the cinnamon and stir to incorporate; set aside.

In a large microwave-safe bowl, melt the butter, about 1 minute. Let cool slightly, then add the egg, brown sugar, oil, vanilla, and whisk to combine. Add the buttermilk and whisk again.

Add the flour, nutmeg, baking soda, and salt, stirring until just combined. Batter will be thick and lumpy.

Pour half of the batter into the prepared pan, then sprinkle 75% of the cinnamon-sugar mixture evenly over the surface. Top with remaining batter, then sprinkle the rest of the cinnamon-sugar mixture.

Bake for 50 minutes or until a toothpick inserted in the center comes out clean. Cool in the pan for 15 minutes before inverting.

Invert the bread and cool completely on a wire rack before slicing and serving.