Preheat your oven to 350°F (175°C). Grease or line a muffin tin with paper liners.

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

In another bowl, mix the buttermilk, melted butter, eggs, and vanilla extract until smooth.

Pour the wet ingredients into the dry ingredients and gently fold using a rubber spatula until just combined. Be careful not to overmix; some lumps are okay.

In a small bowl, combine the brown sugar, ground cinnamon, and softened butter. Mix until it forms a paste.

Spoon a small amount of muffin batter into each muffin cup, filling them about ⅓ full.

Add a generous teaspoon of the cinnamon filling on top of the batter in each cup.

Top with more muffin batter until the cups are about ⅔ full, ensuring the filling is covered.

Using a toothpick or knife, swirl the batter gently to create a marbled effect.

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

Remove from the oven and allow to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.

In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth. Adjust the consistency with more milk if necessary.

Drizzle the glaze over the cooled muffins.