Prepare the base: Working on a flat surface, draw a circle in the center of plywood base that is 2 to 3 inches smaller than the largest cake tier. Using a large offset spatula, spread royal icing over surface between the drawn circle and edge of round plywood board to coat surface smoothly. Tap board on work surface to smooth and eliminate any bubbles. Set aside to dry, about 24 hours. When royal icing is dry, glue ribbon to edge of board, wrapping around to cover entirely.
Cut foam board into rounds, one each of the following diameters: 18, 12, and 6 inches. When cakes have completely cooled, place one of each size cake right side up on its corresponding size foam-board round. Place remaining three tiers on corresponding size cardboard cake rounds. Wrap all cakes tightly in plastic, and chill for at least 6 hours. (This makes the layers firm and easy to handle.) Cakes can also be frozen at this point.
Remove cakes from refrigerator. If cakes are not level, use a serrated knife to trim the tops off, making sure to maintain a 2-inch height on each cake. Place one of each cake on the corresponding size foam-board rounds, cut side up. Brush the cut surface of the bottom base cake layers generously with lemon simple syrup.
Using a medium-large pastry bag fitted with a coupler, pipe soft Italian meringue around the perimeter of each cake that has been moistened with syrup. This "dam" will prevent the lemon curd from seeping out. Fill the 18-inch cake layer with 4 2/3 cups lemon curd, the 12-inch base cake layer with 2 1/3 cups lemon curd, and the 6-inch cake layer with 1/2 cup lemon curd. Place the corresponding cake layers on top of filled layers, cut side down. Brush top layers generously with lemon simple syrup.
Using an offset spatula to give each tier a "crumb coat," ice top and sides of cake with a thin coat of soft Italian meringue. This thin layer of icing will seal the cake. Start from the center, and work toward and over the edge, making sure to spread the meringue over the sides of the tier. Let dry at room temperature for 30 minutes.
To ice the cakes: Using a clean offset spatula, spread a smooth, even layer of soft Italian meringue, to coat cake entirely. Be sure to cover all exposed cake. The icing should be smooth and uniform.
Place the largest tier on the serving platter. Insert four columns, narrow side down, into the 18-inch tier; each column should be 4 inches from the edge of the cake and evenly spaced around the cake. Fill a large pastry bag, fitted with a large star (#869) tip, with a freshly made batch of meringue. Beginning from the center, pipe 2 1/2-inch cones to cover the entire top of tier.
Insert columns into the 12-inch tier; pipe 12-, and 6-inch tiers in the same manner as in step 7. When ready to assemble the cake, dab a small amount of meringue onto the top of each column to fasten it to the above tier. Carefully stack the tiers.
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