Coarsely chop 1/2 cup walnuts. Reserve remaining walnut halves.
Preheat oven to 350°F
Lightly coat a 5-by-9-inch loaf pan with non-stick cooking spray.
In a large bowl, with mixer on medium-high speed, beat to blend egg, buttermilk, and butter. Add chopped walnuts, flour, brown sugar, baking soda and powder, salt and vanilla; stir just until evenly moistened.
Pour into loaf pan and arrange remaining walnut halves evenly on top of batter.
Bake 45 to 50 minutes, until bread begins to pull away from side of pan. Let bread cool in pan on wire rack for 10 minutes. Run a knife between bread and pan sides, then turn out onto wire rack. Let cool 10 minutes.