Preheat the oven to 350°. For the cake, cream together the butter and sugar until light and fluffy and then mix in the eggs. Blend the yogurt with the buttermilk. Sift together the dry ingredients and add them alternately with the yogurt mixture into the egg mixture. Stir in the lemon rind and walnuts. Pour the batter into a buttered 9 x 13 inch baking pan and bake for 30-40 minutes or until a toothpick inserted in the center comes out mostly clean. The cake should still be moist.
Meanwhile, make the glaze by simmering together all the ingredients, covered, for about 15 minutes. When the cake is done, remove it from the oven, turn the oven off, pour the glaze over the hot cake, and return it to the oven for about 10 minutes.
This recipe, published in Sundays at Moosewood Restaurant (ISBN 0-671-67989-9) freezes well and makes a nice change from the usual breakfast coffee cakes.