Preheat your oven to 350°F (175°C) and grease a muffin tin or line it with muffin liners.
In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, and granulated sugar until well combined.
In another bowl, whisk together the buttermilk, melted butter, eggs, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and gently fold them together using a rubber spatula until just combined. Be careful not to overmix; a few lumps are okay.
Make the Cinnamon Filling
In a small bowl, mix together the brown sugar, ground cinnamon, and softened butter until it forms a paste.
Assemble the Muffins
Scoop about 1 tablespoon of muffin batter into each muffin cup to create a base layer.
Add about 1 teaspoon of the cinnamon filling on top of the batter in each cup.
Top with another tablespoon of muffin batter to cover the filling.
Using a toothpick or knife, gently swirl the batter to create a marbled effect, but do not overmix.
Bake the Muffins
Bake in the preheated oven for 15-18 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a cooling rack.
Prepare the Glaze
In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth. Adjust the milk as needed for desired consistency.
Drizzle the glaze over the cooled muffins before serving.
Notes
For best results, ensure all ingredients are at room temperature. These muffins are best enjoyed fresh but can be stored in an airtight container for up to 3 days.