Preheat oven to 325ºF. Butter and flour a 9x5-inch loaf pan.
In a large bowl, cream butter. Add sugar and beat until light and fluffy. Add eggs one at a time, mixing after each addition. Add salt, vanilla and zest; mix again. Scrape sides of bowl. Continue mixing slowly while gradually adding in all the flour. Scrape sides of bowl, then beat again on medium speed for 10-15 seconds, until smooth. Be careful not to over-mix. Batter will be very thick. Spoon batter into pan and smooth top evenly.
Bake at 325ºF. for 1 hour, or until toothpick inserted into center comes out clean. Let the cake cool completely before slicing or serving. Drizzle with a glaze when cool if desired.
Simple Vanilla Glaze: Stir together 1 cup powdered sugar, 1 tsp. vanilla extract, adding 3-4 Tbsp. milk or water until you have a smooth, pourable glaze. Pour immediately; glaze will harden as it dries. Citrus glaze: Follow instructions above, but use 1 tsp. fresh lemon or orange zest instead of vanilla, and use 3-4 Tbsp. lemon or orange juice instead of milk or water.