The bright color and flavor of passion fruit enhances the rum-syrup infused tiers of this wedding cake; the layer of rum-and-vanilla buttercream beneath the fondant adds richness. Serve with a slice of fruit such as star fruit, and coconut sorbet.
Trim tops of cakes to be level and 2 inches high. Cut each cake in half horizontally to create 2 layers from each.
To assemble each tier, set bottom layer on foam board. Brush a generous amount of rum syrup on top of layer, soaking surface. To create a rim for the passion-fruit curd, pipe a line of buttercream around top edge using a round 1/4-inch (such as Ateco number 11) pastry tip. Use an offset spatula to spread a thin layer of curd over cake, spreading to the buttercream rim. Gently set next layer on top. Repeat for all but last layer (7-inch tier has 6 layers; others have 4); on last tier, brush only rum syrup on top. Spread a thin layer of buttercream on top and sides of assembled tier to seal in crumbs. Refrigerate until set, about 15 minutes. Spread another thin layer of buttercream; refrigerate.
When buttercream is set, coat tiers with rolled fondant (see fondant amounts, above). For each tier, roll fondant on a clean work surface lightly dusted with cornstarch until 1/8 inch thick and large enough to cover tier. Dust a straw mat with cornstarch, lay it on fondant, and roll rolling pin firmly across it; repeat to cover all of fondant that will show on sides and top edge of tier. Drape fondant over rolling pin; center and place it on tier. Starting at the top, smooth fondant onto cake with your hands, taking care not to mar the texture. Cut away excess with a pastry wheel. Do not refrigerate finished tiers.
Reinforce tiers: Insert 12 dowels vertically into the 16-inch tier, placing 4 in the center and the rest evenly spaced 2 inches from edge. Insert 9 dowels into the 14-inch tier and 7 into the 10-inch, placing 1 in the center, the rest 3 inches from edge.
When ready to assemble cake, place cake mat on cake platform; center bottom tier on top. Spread curd on top of tier with an offset spatula. Center and stack remaining tiers, spreading each with curd before adding next; do not coat top tier. Decorate top of cake with fruit and flowers.
Use fruits and flowers grown without pesticides to top this cake. You will need a clean straw mat for texturing the fondant.