Prep: 10 minutes; Bake: 20 minutes; Cool: 10 minutes.
1. Preheat oven to 350°. Line a muffin pan with paper baking cups.
2. Combine 1/2 cup flour, light brown sugar, and 1/2 teaspoon cinnamon in a small bowl. Cut in 4 tablespoons butter with a pastry blender until streusel is crumbly; set aside.
3. Sift together remaining 2 cups flour, remaining 1 teaspoon cinnamon, baking powder, and kosher salt in a medium bowl. Mix sugar and remaining 10 tablespoons butter in a separate bowl at medium speed with an electric mixer until well combined. Beat eggs, 1 at a time, into butter mixture until blended. Add flour mixture alternately with milk, mixing until smooth. Add vanilla, and mix well.
4. Divide batter evenly among baking cups, filling each about three-quarters full. Sprinkle evenly with reserved streusel topping.
5. Bake at 350° for 20 to 25 minutes or until springy and a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack. Dust with powdered sugar before serving, if desired.
*Remove eggs from refrigerator, and place in a bowl for 30 minutes.