Special equipment: two 8-inch round cake pans, two 6-inch round cake pans, a large piping bag with a 1/2-inch opening, 1-inch and 2-inch new (unused) craft brushes, culinary squeeze bottles
For the cakes: Preheat the oven to 350 degrees F. Grease two 8-inch and two 6-inch round cake pans with flour-based baking spray.
Sift together the flour, baking powder and salt in a bowl. Whisk together the egg whites and milk in a separate bowl.
Beat the butter in the bowl of an electric mixer using the paddle attachment until soft and creamy. Add the granulated sugar gradually and beat until light and fluffy. Scrape down the bowl intermittently. Beat in the vanilla. Add the flour mixture alternately with the egg white and milk mixture, beginning and ending with the flour mixture. Mix until smooth on medium-low speed after each addition.
Divide the batter among the prepared pans. Bake until the cakes spring back when pressed in their centers, about 25 minutes for the 6-inch cakes and 35 minutes for the 8-inch cakes. Allow the cakes to cool slightly in the pans and then turn out on wire racks to cool completely.
When cool, level the cakes using a large serrated knife or cake leveler.
For the buttercream: Cream the butter and confectioners' sugar together on low speed in the bowl of an electric mixer. When the mixture is combined, increase the speed to high and whip until light in color. Turn the mixer off and add the vanilla extract and 1/3 cup heavy cream. Beat again until the frosting is light and fluffy. Add additional heavy cream if the mixture is too thick to spread or pipe.
Transfer the frosting to a large piping bag with a 1/2-inch opening. Pipe a line of frosting around the top edge of one of the leveled 8-inch cakes. Spoon lemon curd in the center and spread evenly. Place the second cake layer on top of the filled cake and cover with frosting. Spread the buttercream as evenly as possible and refrigerate until the frosting is firm, about 1 hour. Repeat this process with the 6-inch cakes. Reserve the leftover buttercream for later use.
When both cakes are well chilled and the frosting is firm, use a large spatula to transfer the smaller cake to the top center of the larger cake. Keep the cake chilled
For the candy brushstrokes and decor: Place a sheet of parchment paper over a large baking sheet. Place the different colors of wafers in separate microwave-safe bowls and heat each in the microwave at 100 percent power until they can be stirred smooth, about 30 seconds for 6 ounces of candy. In separate bowls, combine different colors of the candy to create new colors. Make as many hues as desired.
Use a spoon to place quarter-sized dollops of the candy on the parchment sheet. Hold the parchment sheet steady with one hand as you place a craft brush over the melted candy. Use quick motions to brush the candy upward, resulting in one long candy brushstroke. Repeat the process with the other colors of candy. Transfer the brushed candy to the refrigerator to chill, 5 to 7 minutes.
When firm, transfer the chilled candy to a large plate. Repeat the candy brushing process until you have about 40 candy brushstrokes for decorating the cake.
Transfer any leftover melted candy to squeeze bottles and drizzle decoratively over one side of both cake tiers.
Use leftover buttercream to attach the firm candy brushstrokes to the opposite side of the cake; arrange them upright on both tiers. Press a few small brushstrokes into the frosting on top of the cake so they also stand upright. Use as many candy brushstrokes as possible to fill up one side of the cake.
Before serving, arrange fresh sliced citrus on the edges of the candy-drizzled side of the cake. Tuck fresh mint between the fruit and candy brushstrokes. Present to delighted guests.
To serve, move aside the fruit slices and cut the cake into pieces. Serve the fruit slices alongside pieces of cake, if desired.
Tips for success: If at any time the candy begins to harden, reheat it in 10-second intervals in the microwave until melted. Use different widths of craft brushes for varied brush strokes. Buy several craft brushes so you won't have to wash and dry one brush between uses. They are inexpensive and can usually be purchased in bags of 6 or more. Pat the fresh citrus slices dry with paper towels before applying to the sides of the cake.