Heat oven to 325 degrees. Butter and flour two 8- by 2-inch square cake pans and line with parchment paper. Cream together the butter and sugar in the bowl of an electric mixer fitted with the paddle attachment until light and fluffy. Add the eggs, one at a time, beating well after each addition. Sift together flour, salt, and baking powder twice. Add to egg mixture, alternating with milk, starting and ending with flour. Add vanilla and stir just until thoroughly blended. Gently fold in the zest. Pour the batter into prepared pans and bake for 90 minutes; cakes are done when a tester inserted into the center of cakes comes out clean. Let cool in pans for about 10 minutes. Turn out onto a wire rack, remove parchment, and cool completely. Meanwhile, prepare Meringue Buttercream: In a small saucepan over medium heat, bring sugar and 1/3 cup water to a boil. Boil until syrup reaches soft-ball stage (238 degrees on a candy thermometer). Meanwhile, beat egg whites on low until foamy. Add cream of tartar; beat on medium-high until stiff but not dry. With mixer running, pour syrup into egg whites in a steady stream, and beat on high speed until completely cool, about 15 to 20 minutes. Meanwhile, cream butter in a separate bowl to soften. Beat in butter gradually. Add vanilla, beat 2 to 3 minutes, until frosting is smooth and spreadable. If it looks curdled during the beating process, continue beating until smooth. Once the cakes have cooled, trim each so that they are level and even. Each trimmed cake should be about 2 inches high. Brush the top and sides of each cake with a pastry brush to remove crumbs. The bottom tier will need a sturdy base. Trace a 6-inch square onto a piece of foam board 3/16-inch thick (available from art supply stores). Cut out the square with a utility knife. Cut one cake into a 6- by 6-inch square. Set aside, reserving the leftover cake for another use. Place cake on foam board. Cut the second cake into 2 smaller squares, one 3 3/4- by 3 3/4-inches and the second into a square 2 1/4- by 2 1/4-inches. Once again, reserve the leftover cake for another use. Brush the top and sides of each cake to remove crumbs. Using an offset spatula, ice top of the 6-inch-square cake with buttercream to give the tier a "crumb coat, " a thin layer of icing that seals the cake. Start from the center and work out, making sure to push the buttercream over the sides of the cake. Smooth the icing, and chill in the refrigerator to set until smooth to the touch, about 30 minutes to one hour, until it doesn't stick when you touch it. Repeat with the remaining cakes. Assemble the cake by placing the tiers on top of one another in descending order of size. Once assembled, give the tiers a final coat of buttercream. Use an offset spatula to smooth the icing. (Dipping the offset spatula in warm water, then wiping it off again, will make a clean finished edge.) Reserve extra buttercream for decorating. Chill in the refrigerator to set the icing, about 30 minutes to 1 hour. Once the final coat sets, fill a pastry bag with the reserved buttercream; fit with a round #3 pastry tip and pipe a decorative border along the edges of each tier. Makes 1 three-tiered cake.