Begin by preheating your oven to 350°F (175°C). Prepare a muffin tin by lining it with paper liners or greasing each cup generously to prevent sticking.
In a large mixing bowl, thoroughly whisk together the all-purpose flour, baking powder, salt, granulated sugar, and 1 teaspoon of ground cinnamon until well combined.
In a separate bowl, blend the melted unsalted butter, milk, large egg, and vanilla extract. Whisk these wet ingredients until they are smooth and cohesive.
Gently pour the wet mixture into the bowl with the dry ingredients. Fold the mixture together carefully, mixing until just combined. It’s important to avoid overmixing, so don’t worry if there are a few lumps remaining.
In a small bowl, prepare the filling by combining the packed brown sugar, 1 tablespoon of ground cinnamon, and chopped nuts (if desired). Mix until evenly distributed.
Scoop the muffin batter into each muffin cup, filling them halfway to allow space for the filling. Evenly sprinkle a spoonful of the prepared filling mixture over the batter in each cup. Then, top the filling with additional batter until each cup is about 3/4 full.
Using a toothpick or a small knife, gently swirl the filling into the batter, creating a marbled effect without fully blending.
Place the muffin tin in the preheated oven and bake for 18-20 minutes. Test for doneness by inserting a toothpick into the center; it should come out clean when the muffins are ready.
While the muffins are baking, whip up the icing by mixing the powdered sugar with just enough milk to create a smooth, drizzling consistency. Adjust the milk as necessary to reach your desired thickness.
Once the muffins are baked, allow them to cool in the pan for about 5 minutes. Afterward, carefully transfer them to a wire rack. While they are still warm, drizzle the prepared icing generously over the top of each muffin.
Notes
Serve on a decorative platter and garnish with extra cinnamon and nuts for added appeal.