Preheat oven to 350°. Place a rack in the center of the oven. Spray a 9 x 5 loaf pan with baking spray (or butter and flour) and set aside.
To prepare streusel, combine first 5 ingredients in a medium bowl. Add 2 tablespoons melted butter, stirring until well combined. Stir in nuts. Set aside.
In a large bowl, whisk together flour, baking soda, baking powder, and 1/2 teaspoon salt in a bowl.
In another bowl, beat 5 tablespoons butter and granulated sugar on medium-high speed until well blended.
Add eggs, one at a time, beating well after each addition; beat in vanilla.
Beating at low speed, add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture; beat just until combined.
Scrape half of batter into a 9 x 5-inch loaf pan coated with baking spray; sprinkle with half of streusel mixture.
Spread remaining batter over streusel; swirl.
Sprinkle remaining streusel on top of batter.
Bake at 350° for 50 minutes or until a wooden pick inserted in center comes out with moist crumbs clinging. Cool 10 minutes in pan on a wire rack. Remove from pan; cool completely on wire rack.