Preheat your oven to 350°F. Grease a standard loaf pan or line it with parchment paper.
In a mixing bowl, whisk together the melted butter and sugar until combined.
Add the egg, buttermilk, and vanilla. Mix until smooth.
In another bowl, combine flour, baking powder, baking soda, salt, and cinnamon.
Slowly add the dry ingredients into the wet, stirring just until everything is incorporated. Don’t overmix.
Pour the batter into your loaf pan and smooth the top.
Bake for 45–50 minutes, or until a toothpick comes out clean from the center.
Let the loaf cool for 10 minutes in the pan, then transfer to a wire rack.
Mix the cinnamon and sugar topping together in a small bowl.
Brush the loaf with melted butter, then sprinkle (or generously coat) with the cinnamon sugar.
Slice and serve while slightly warm, or let it cool completely—it’s delicious either way.