Preheat oven to 375 degrees F. (See note for different types of muffin tins.) Place muffin liners in a 12-cup muffin tin. This recipe makes 15-16 muffins, so use a second muffin tin for the remaining muffins or bake in two batches.
For the muffins, in a medium bowl, whisk together the sugar, butter, eggs, sour cream, milk and vanilla until evenly combined.
Add the flour, baking powder, baking soda and salt. Whisk until just combined and no dry streaks remain. Don't over mix. Set aside.
For the cinnamon streusel, in a small bowl, stir together the flour, brown sugar and cinnamon. Add the melted butter and mix until combined and the streusel clumps into small pieces.
Scoop about 2 tablespoons muffin batter into each muffin liner (a #40 cookie scoop works great). Crumble the cinnamon streusel to break into small pieces. Sprinkle 2 to 3 teaspoons streusel over the batter in each muffin cup.
Scoop another 2 tablespoons (see note) batter on top of the cinnamon streusel. Sprinkle with a bit more streusel.
Bake for 15 to 17 minutes until the top of the muffins spring back lightly to the touch.
Remove the muffins to a cooling rack to cool completely.
For the glaze, in a small bowl, add the softened cream cheese, butter and vanilla. Mix with an electric mixer until light and creamy. Add the powdered sugar and mix on low speed until fully combined, scraping down the side of the bowl as needed. Add the milk and mix until well combined. The consistency should be thick but pourable. Add additional milk (or powdered sugar) to adjust the consistency, if needed.
Drizzle the glaze evenly over the cooled muffins.
Notes
Muffin Tin:I highly recommend using a muffin tin that is light metal vs a dark-coated muffin tin. If you only have a dark-coated muffin tin, reduce the oven temperature to 350 degrees F. Batter Amount:the muffin cups for this recipe are filled about 3/4 full. If you live at high altitude or are using a muffin tin with more shallow muffin cups, use a bit less batter per cup than stated in the recipe. I use the USA brand of muffin tins for reference. Sour Cream:1 cup buttermilk can be subbed for the sour cream and milk in the recipe.