Preheat oven to 350°F. Butter and flour a 9-inch square baking pan. In large bowl, beat softened butter, brown sugar and zest until fluffy. Add eggs and beat batter until blended.

In a separate bowl, sift together flour and remaining dry cake ingredients. Blend into batter at low speed one third of dry mix and half of buttermilk, alternately, until both have been mixed in. Spread batter in pan. Bake until toothpick inserted in center comes out clean, 25 to 30 minutes.Remove from oven and let cool on rack 10 minutes. Loosen cake from pan with slender spatula, and invert gently onto rack, then re-invert onto another rack and allow to cool completely before frosting.

Set medium saucepan over medium-low heat. Cook butter until deep golden brown, swirling occasionally, 10 to 15 minutes. Pour into a mixer bowl and allow to cool.

Chill butter bowl until butter is firm enough to beat, about 30 minutes. Add remaining frosting ingredients, except for nuts, and beat until fluffy, 1 to 2 minutes.

Split cake horizontally with long serrated knife. Lift off top layer with 2 large spatulas and carefully set aside. Set bottom layer on platter, and spread upper surface with half of frosting. Replace top layer, and spread upper surface with remaining frosting. Sprinkle with nuts.

Notes:

Recipe and photo from Sunset Magazine, October 2010.