In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in buttermilk and vanilla. In another bowl, whisk flour, baking powder and salt. Add to creamed mixture stirring just until moistened. Gently fold in blueberries.
Fill greased or paper-lined jumbo muffin cups two-thirds full.
Mix topping ingredients; sprinkle over tops. Bake 20-25 minutes or until a toothpick inserted in center comes out clean. Cool 5 minutes before removing from pan to a wire rack. Serve warm with butter.
For standard-size muffins: Make batter as directed; fill greased or paper-lined standard muffin cups two-thirds full. Bake in a preheated 400° oven for 15-20 minutes or until a toothpick comes out clean.