• Preheat the oven to 350 degrees Fahrenheit. Line two muffin tins with cupcake liners. Set aside.

To Make Streusel Topping

• In a small bowl, whisk together the dry ingredients for the streusel.
  1/2 cup brown sugar, 1/2 cup all-purpose flour, 1/2 teaspoon cinnamon
• Using a pastry cutter or a fork, cut the butter into the dry ingredients until the mixture is crumbly. Set aside.
  4 tablespoons unsalted butter

To Make the Filling

• Whisk together the ingredients for the filling and set aside.
  1/4 cup brown sugar, 1/4 cup flour, 1/2 teaspoon cinnamon

To Make Batter

• In a large bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt. Set aside.
  2 cups all-purpose flour, 2 teaspoons baking powder, 1 teaspoon baking soda, 1 teaspoon cinnamon, 1/2 teaspoon salt
• In a stand mixer fitted with the paddle attachment or in a large bowl using a hand mixer, beat the butter and granulated sugar together until light and fluffy, about 3 to 6 minutes.
  1/2 cup unsalted butter, 1 cup granulated sugar
• Add in the eggs, one at a time. Now beat in the vanilla extract. Scrape down the sides and beat in the sour cream until the mixture is smooth and combined. Scrape down the sides again. Finally, beat in the milk.
  3 eggs, 2 teaspoons vanilla extract, 3/4 cup sour cream, 1/4 cup milk
• Add in the flour mixture, and beat on low for a few moments until you can beat on high until the mixture is just combined.

Putting Muffins Together

• Add about a tablespoon of the batter to the bottoms of the prepared muffin tins. Add 1 teaspoon of filling to the muffins. Then, top with the remaining batter. Finally, top with the streusel topping.
• Bake for 20 to 25 minutes or until a toothpick inserted in a muffin in the middle of the pan comes out with just a few crumbs. Take the muffins out of the oven and after a few minutes transfer them to a cooling rack to cool completely.

Icing

• Whisk together the powdered sugar and milk. Drizzle it over the top of the cooled muffins.
  1/2 cup powdered sugar, 2 teaspoons milk