Preheat your oven to 350℉ (175℃) and prepare a muffin tin with large bakery-style liners.

In a medium bowl, combine brown sugar, flour, and ground cinnamon. Drizzle in melted butter and mix until crumbly.

Cream together softened butter and white sugar until light and fluffy. Add vanilla, eggs, milk, and sourdough starter; mix until smooth.

Gradually add baking powder and all-purpose flour to the wet mixture, stirring gently to combine.

Spoon batter into liners halfway, layer with cinnamon sugar, add more batter, and top with remaining cinnamon sugar.

Bake for 25-30 minutes until golden brown; a toothpick should come out clean.

Prepare glaze by whisking powdered sugar and milk to desired consistency.

Once muffins cool slightly, drizzle glaze over tops and serve warm with coffee or tea.