1. Preheat the Oven:
   Preheat your oven to 350°F (175°C). Grease and flour a 9×5-inch loaf pan or line it with parchment paper for easy removal.
2. Mix the Dry Ingredients:
   In a medium bowl, whisk together the flour, baking powder, salt, and granulated sugar.
3. Mix the Wet Ingredients:
   In a separate bowl, whisk together the milk, melted butter, eggs, and vanilla extract until well combined.
4. Combine the Wet and Dry Ingredients:
   Add the wet ingredients to the dry ingredients and stir until just combined. Be careful not to overmix, as this can make the bread dense.
5. Prepare the Cinnamon-Sugar Swirl:
   In a small bowl, mix together the brown sugar and cinnamon until evenly combined.
6. Assemble the Bread:
   Pour half of the bread batter into the prepared loaf pan and spread it evenly. Sprinkle half of the cinnamon-sugar mixture over the batter. Then, pour the remaining batter on top and spread it evenly again. Sprinkle the rest of the cinnamon-sugar mixture on top.
7. Swirl the Cinnamon:
   Using a butter knife or skewer, gently swirl the cinnamon-sugar mixture into the batter to create a marbled effect.
8. Bake the Bread:
   Bake in the preheated oven for 40-50 minutes, or until a toothpick inserted into the center comes out clean. The top of the bread should be golden brown, and the bread should feel firm to the touch.
9. Cool the Bread:
   Let the bread cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.

For the Icing:

1. Make the Icing:
   While the bread is cooling, whisk together the powdered sugar, melted butter, milk (or heavy cream), vanilla extract, and a pinch of salt until smooth and creamy. If the icing is too thick, add a little more milk; if it’s too thin, add a bit more powdered sugar.
2. Drizzle the Icing:
   Once the bread has cooled slightly (it can still be a little warm), drizzle the icing over the top of the bread, letting it drip down the sides.
3. Serve and Enjoy:
   Slice the bread and serve it warm or at room temperature. Enjoy the sweet, cinnamon-y goodness with a cup of coffee or tea!