Preheat oven to 375°.
To prepare streusel, combine first 4 ingredients in a small bowl; set aside.
To prepare muffins, lightly spoon 2 cups flour into dry measuring cups; level with a knife. Combine 2 cups flour, 3/4 cup sugar, baking powder, baking soda, and salt in a medium bowl, stirring with a whisk. Make a well in center of mixture. Combine buttermilk and remaining ingredients except cooking spray in a small bowl; add to flour mixture, stirring just until moist. Spoon batter into 15 muffin cups coated with cooking spray. Sprinkle streusel evenly over batter. Bake at 375° for 18 minutes or until golden brown. Remove muffins from pans immediately; place on a wire rack.