Preheat oven to 350
Combine flour, baking powder, and salt in a bowl and set aside
In a larger mixing bowl, which sugar and melted butter together
Add vanilla, egg, and milk, and which until combined
Stir in dry ingredients until just combined
Make the filling by stirring the brown sugar and cinnamon together
Then stir in melted butter until thick and crumbly
Divide half the muffin batter between 12 muffin cups
Sprinkle with half the filling mixture
Repeat by topping the filling mixture with the second half of the muffin batter, then ending by topping the muffins with the second half of the filling
Bake in center of oven for 15-18 minutes, until the muffins begin to brown on the edges
The toothpick test doesn’t really work on these because of the gooey filling
Remove from oven and let them cool 5-10 minute before removing from pan
While the muffins are cooling, stir icing ingredients together until smooth
Drizzle icing over muffins
Serve hot or cold
Note: You will want to remove these from the pan while they’re warm in case any sugar mixture leaked out during baking. When it cools it can act as glue and make it hard to remove them.