Preheat oven to 350F and line 12 slots in a muffin tin with liners.

In a large mixing bowl whisk together sour cream, milk, eggs, sugar, vegetable oil and vanilla. Add flour, baking soda, baking powder and cinnamon to the bowl and fold in with a flat spatula.

Scoop about ¼ cup of batter into each of the 12 muffin slots and set aside.

To prepare crumb topping, combine brown sugar, all purpose flour, cinnamon and salt in a small mixing bowl. Place softened butter in the bowl and use the back of a fork to press butter into dry ingredients. Continue mixing with the fork until the butter is fully incorporated into the dry ingredients and a sand like mixture remains.

Separate crumb topping evenly over batter in the muffin tin - about 1.5 tbsp per muffin.

Place muffin tin in the oven and bake for 26-28 minutes. To check that the muffins are done, insert a toothpick into the center of the muffin and check that it is clean when it comes out.

Allow muffins to cool completely. Prepare the icing by mixing powdered sugar with milk and drizzle over completely cooled muffins