01 - Preheat the oven to 350°F. Grease an 8x4-inch loaf pan and line it with parchment paper, leaving an overhang on the sides for easy removal.
02 - In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, salt, and ground cinnamon until evenly distributed.
03 - In a separate bowl, whisk together the milk, eggs, melted butter, and vanilla extract until smooth and well blended.
04 - Pour the wet mixture into the dry ingredients and fold gently with a spatula until just combined. Be careful not to overmix, as this can make the loaf tough.
05 - In a small bowl, stir together the brown sugar and ground cinnamon until uniformly mixed. Set aside.
06 - Pour half of the batter into the prepared loaf pan and spread evenly. Sprinkle the cinnamon sugar filling in an even layer across the surface. Pour the remaining batter on top and smooth with a spatula.
07 - Bake on the center rack for 40 to 50 minutes, or until a toothpick inserted into the center of the loaf comes out clean. If the top browns too quickly, tent loosely with aluminum foil during the last 10 minutes.
08 - Let the loaf cool in the pan for 10 minutes, then transfer to a wire rack. While still warm, brush the top evenly with melted butter and sprinkle generously with the cinnamon sugar topping mixture.
09 - Allow the loaf to cool completely before slicing into even portions. Serve at room temperature.