Cinnamon Streusel Coffee Cake
In a medium bowl stir together 1/2 cup each packed brown sugar and all-purpose flour and 1 tsp. ground cinnamon. Cut in 1/4 cup butter until mixture resembles coarse crumbs.
Powdered Sugar Icing
In a small bowl stir together 1 1/4 cups powdered sugar, 1 Tbsp. milk, and 1/2 tsp. vanilla. Stir in additional milk, 1 tsp. at a time, to reach drizzling consistency.
To make 3/4 cup sour milk, place 2 tsp. lemon juice or vinegar in a glass measuring cup. Add enough milk to make 3/4 cup total liquid; stir. Let stand 5 minutes before using.