Preheat oven to 350 degrees. Butter and flour a jumbo 12-cup muffin tin or line with baking cups.

Prepare Streusel Topping: In a medium bowl, stir together brown sugar, flour, salt and cinnamon. With a pastry blender or fork, cut in butter until mixture resembles large coarse crumbs; refrigerate until ready to use.

Prepare Muffins: In a small bowl, whisk together flour, baking powder, and baking soda; set aside. In a large bowl, using an electric mixer on medium speed, beat together butter, sour cream, granulated sugar, and vanilla until light and 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, and 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. Dust with confectioners' sugar, if desired.