Preheat the oven to 180C/350F. Line a 12-count muffin tin with muffin liners.
In a small bowl, whisk the milk and apple cider vinegar and let sit for 5 minutes, to curdle.
In a large mixing bowl, add the flour, sugar, baking powder and salt. Add the curdled milk mixture, oil, and vanilla extract. Stir until no large clumps remain. Fill each muffin until half filled.
In a small bowl, mix hot melted butter, cinnamon and brown sugar.
Add 1/2 teaspoon of the mix into each muffin and swirl it a few times into the batter with a fork, careful not to let it completely mix. Add the remaining muffin batter.
With the remaining cinnamon swirl mix, drizzle on top of each muffin and use a skewer to swirl through.
Bake the muffins for 18-20 minutes, or until a skewer comes out mostly clean.
Remove the muffins from the oven and let them cool completely. While cooling, prepare the glaze. Drizzle a spoonful over each muffin and let it set.
* For vegan muffins, use vegan butter.TO STORE: Leftover muffins can be stored at room temperature, covered, for up to five days. If you'd like them to keep longer, store them in the refrigerator. TO FREEZE: Place the muffins in a ziplock bag and store them in the freezer for up to six months.