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

In a separate bowl, whisk together the milk, oil, egg, and vanilla extract.

Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix! A few lumps are okay.

In a small bowl, combine the melted butter, brown sugar, cinnamon, and flour. Mix until a paste forms.

Fill each muffin cup about halfway with batter.

Drop spoonfuls of the cinnamon swirl mixture on top of the batter in each muffin cup. Use a toothpick or skewer to swirl the cinnamon mixture into the batter.

Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean and the edges are golden brown.

Let the muffins cool in the muffin tin for a few minutes before transferring them to a wire rack to cool completely.

While the muffins are cooling, whisk together the powdered sugar, milk, and vanilla extract in a small bowl until smooth and creamy. Add more milk, 1 tablespoon at a time, until you reach your desired consistency.

Once the muffins are cooled, drizzle the icing over the top. Serve immediately.