Prepare streusel topping: in a medium bowl, stir together brown sugar, flour, salt + cinnamon. with a pastry blender or two knives, cut in butter until mixture resembles large coarse crumbs; refrigerate until ready to use.
Preheat oven to 350 degrees.
Prepare muffins: butter + flour a jumbo 12-cup muffin tin (or two 6-cup muffin tins).
In a small bowl, whisk together flour, baking powder, baking soda + cinnamon; set aside.
Using an electric mixer on medium speed, beat together butter, sour cream, granulated sugar, + vanilla until light + fluffy.
Beat in eggs, one at a time, until well combined.
With mixer on low, beat flour mixture into butter mixture.
Divide half the batter among prepared muffin cups.
Top with half the streusel mixture, then remaining batter, + top with streusel.
Bake 25 to 30 minutes or until a toothpick inserted in center of a muffin comes out clean.
Cool in pan 5 minutes, then transfer to a wire rack to cool completely.