Preheat the oven to 350°F (175°C) and grease a muffin tin.

In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.

In another bowl, mix the buttermilk, melted butter, and eggs until combined.

Pour the wet ingredients into the dry ingredients and stir until just combined.

In a small bowl, mix together the brown sugar, cinnamon, and softened butter until well combined.

Spoon a small amount of muffin batter into each muffin cup, followed by a spoonful of the cinnamon filling, and then top with more muffin batter until cups are about 2/3 full.

Use a toothpick or knife to swirl the filling into the batter slightly.

Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center comes out clean.

Let the muffins cool in the pan for 5 minutes before transferring to a wire rack.

In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth.

Drizzle the glaze over the cooled muffins before serving.