Cinna-full Coffee Cake Muffins

2 1/2 cups all-purpose flour, divided
1/2 cup dark brown sugar, firmly packed
14 tablespoons unsalted butter, cut into pieces, softened and divided
1 1/2 teaspoons ground cinnamon, divided
1 tablespoon baking powder
1/2 teaspoon kosher salt
3/4 cup granulated sugar
2 large eggs, room temperature
3/4 cup buttermilk
1/2 teaspoon vanilla extract
Confectioners' sugar for dusting

Heat oven to 350 degrees F. Line a muffin pan with paper baking cups.

Combine 1/2 cup flour, dark brown sugar, and 1/2 teaspoon cinnamon in small bowl. Cut in 4 tablespoons butter with pastry blender until streusel is crumbly, then set aside.

Sift together remaining 2 cups flour, remaining 1 teaspoon cinnamon, baking powder and salt in medium bowl. Mix sugar and remaining 10 tablespoons butter in separate bowl at medium speed with electric mixer until well combined. Beat eggs, 1 at a time, into butter mixture until blended. Add flour mixture alternately with buttermilk, mixing until smooth. Add vanilla extract and mix well.

Divide batter evenly among baking cups, filling each about 3/4 full. Sprinkle evenly with reserved streusel topping.

Bake for 20 to 25 minutes or until springy and wooden pick inserted comes out clean. Cool in pan 10 minutes on wire rack. Dust with confectioners' sugar before serving.