John's Jumbo Coffee-Cake Muffins

For Streusel:
1 c. packed dark-brown sugar
1 c. all-purpose flour
1/2 tsp. ground cinnamon
1/2 tsp. salt
1/2 c. cold, unsalted butter, cut into small pieces

For Muffins:
1/2 c. unsalted butter, room temperature, plus more for pans
1 3/4 c. all-purpose flour, plus more for pans
2 tsp. baking powder
1 tsp. baking soda
1 c. sour cream
1 c. granulated sugar
1 tsp. vanilla
2 large eggs
confectioners' sugar, for dusting

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