Prepare streusel topping:
In a medium bowl, mix together brown sugar, flour, salt and cinnamon. then mix the butter ( cut to small pieces) to the rest of materials until mixture resembles large coarse crumbs;put them in fridge until ready to use.
Prepare the muffin:
First preheat the oven to 350F. in a small bowl mix the flour, baking powder and baking soda and set aside. using an electric mixer ( on medium speed) beat together butter, sour cream, sugar and vanilla until light and fluffy. add the eggs one by one until well combined. then put the mixer at low and add the flour to the rest.
I used baking sheets for muffins; if you don't want to use them , grease the pans before loading them. add half of the batter to the muffin pan, top it with a layer of streusel mixture, then put the remaining batter and top it with streusel again.
Bake for 25 to 30 minutes or until a toothpick inserted in center of muffin comes out clean. after they become ready coll them on wire rack for couple of minutes. these muffins are so moist and favorable,I really liked its texture. Hope you try them and enjoy with your tea or coffee.