For the topping: In the bowl of a stand mixer fitted with the paddle attachment, stir together the flour, brown sugar, cinnamon and salt. Add the butter and toss with a fork to coat with the flour mixture. Mix on medium-low speed until the texture resembles coarse cornmeal, with the butter pieces no larger than small peas. Transfer mixture to a medium bowl and place in refrigerator for 30 minutes before using.
Preheat oven to 350F. Line 24 muffin cups with muffin liners or spray with nonstick cooking spray.
For the muffins: In a medium bowl, whisk together flour, baking soda, baking powder and salt; set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream together butter and sugar until light and fluffy. Beat in eggs one at a time,scraping down bowl as needed. Stir in vanilla extract by hand. Add flour mixture and sour cream and stir until just combined.
Divide batter evenly among muffin cups. Sprinkle half of the strudel mixture over the muffins, gently pressing it into the batter. Sprinkle evenly with remaining strudel mixture.
Bake rotating the pans halfway through until golden brown and a toothpick inserted into the center comes out clean, about 20 minutes. Transfer pans to a wire rack to cool completely before removing muffins.
To make milk glaze, whisk the powdered sugar and milk together in a small bowl.
Place muffins on a wire rack set over a baking sheet and drizzle with glaze.