To make streusel mixture, in a food processor or a bowl, whirl or mix 3/4 cup flour, 3/4 cup brown sugar, and 1/4 teaspoon salt. Cut 1/4 cup butter into chunks and add to bowl; pulse mixture or cut in butter with a pastry blender until mixture resembles coarse crumbs. Set aside.
In a bowl, with a mixer on high speed, beat remaining 1/4 cup butter and remaining 1 cup brown sugar until smooth. Beat in eggs and vanilla, scraping down sides of bowl as necessary, until well blended. Beat in mashed bananas.
In a small bowl, stir together remaining 1 1/2 cups flour, baking powder, and remaining 1/4 teaspoon salt. Stir into banana mixture. Gently fold in sour cream until well blended.
Divide batter equally among 6 jumbo (1-cup capacity) or 18 regular (1/3-cup capacity) buttered and floured muffin cups, filling each halfway. Sprinkle streusel evenly over tops of cakes (crumble the streusel first if it sticks together).
Bake in a 350° regular or convection oven until streusel is browned and a wooden skewer inserted into the center of a cake comes out with moist crumbs attached, 18 to 25 minutes. Cool 5 minutes in cups, then invert pan to remove. Serve warm or let cool completely.