Make the Cake: Preheat oven to 350 degrees. Butter two 10-by-2-inch round cake pans, and dust with cocoa. Whisk together flour, salt, and cocoa in a medium bowl; set aside.
Whisk sugar and oil on medium speed in a mixer fitted with the whisk attachment, until combined. Add eggs one at a time, whisking well after each addition. Whisk in food coloring and vanilla. Add flour mixture in 3 batches, alternating with the buttermilk and beginning and ending with flour; whisk well after each addition. Scrape down sides of bowl as needed.
Stir together baking soda and vinegar in a small bowl; add to batter, and beat on medium speed 10 seconds. Pour batter into prepared pans. Bake until cake tester inserted into centers comes out clean, about 40 minutes. Let cool completely in pans on wire racks.
Make the Swiss Meringue Buttercream (our recipe makes 9 cups): Place sugar, egg whites, and salt in a heatproof mixer bowl set over a pot of simmering water. Whisk until sugar dissolves and mixture registers 160 degrees on a candy thermometer.
Attach bowl to the mixer. Whisk on medium speed for 5 minutes. Increase speed to medium-high, and whisk until stiff, glossy peaks form and meringue has cooled, about 6 minutes. Reduce speed to medium, and add butter, 1 piece at a time, whisking well after each addition. Whisk in vanilla. Use immediately, or cover and refrigerate for up to 3 days. (Bring to room temperature, and beat on low speed until smooth before using.)
Assemble the Cake: Trim tops of cakes to be level. Then slice cakes horizontally into three 1/2-inch-high layers (a tad thinner if possible). Lay one cake layer on a piece of foam board, and spread with a thick coating of buttercream (a tad less than 1/2 inch, about 2 cups). Top with a second cake layer, and spread thickly with buttercream. Repeat until you have 5 cake layers and 4 buttercream layers. Spread a thin coat of buttercream over top and sides of tier to seal in crumbs, and chill until ready to cover with fondant. Repeat for the second tier.
To prepare the bottom tier for stacking, insert 1 dowel into the center of the cake (trim it to be level with the top of the icing), and the remaining 6 dowels in a circle 2 inches from the edge of the cake. Set the second tier on top, aligning carefully.
To cover cake, roll out 2 1/2 pounds fondant about 1/4-inch thick. Lay it over the cake, and smooth it down the sides. Trim bottom of fondant with a pizza cutter.
To decorate cake, place it on a decorating turntable. Mix each luster dust with a very small amount of lemon extract to make a paint. Dip a paintbrush into red luster-dust paint, and paint stripes around sides of cake (use painter's masking tape to mark off straight lines; let dry between applications). Use smaller brushes to paint the floral design. If paint starts to dry out, add more lemon extract.(We recruited a fine-art painter to create our floral design; see more of Nathan Stapley's work at nathanstapley.com.)
Cover and refrigerate buttercream for up to 3 days. (Bring to room temperature, and beat on low speed until smooth before using.)
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