Prepare cake boards: Thin 2 cups royal icing to flooding consistency, and spread on top of the 8-, 12-, and 17-inch boards with an offset spatula. Set boards aside to dry, several hours or overnight.
Make bands: Using clear tape, attach 2 poster-board strips so their ends overlap by 1 inch; repeat to make another long band. Trim one to 39 inches long, and the other to 55 inches long. Trim remaining strip to 27 inches long; use double-sided tape to attach it around side of 8-inch cake board, evenly overhanging top and bottom edges. Attach 39-inch band to 12-inch board in the same way. Attach 55-inch band around 17-inch board so band's bottom edge is flush with board's bottom edge.
Decorate separators: Fit a pastry bag with a coupler and an Ateco number 46 basket-weave tip; fill with thick royal icing. Pipe basket-weave pattern on bands; let dry overnight. Hot-glue 3/8-inch-wide ivory grosgrain ribbon around pillars, starting at tops; attach pillars to separators.
Prepare tiers: Place 1 of each size layer onto its corresponding-size foam board; top with a generous layer of mascarpone cream. Cover each with another cake layer. Repeat with more filling and cake to make tiers with 3 layers of cake and 2 of filling. Top with more filling; dust with sugar.
Stack tiers (assemble at reception site): Place 16-inch tier onto prepared 17-inch board. Center 11-inch separator with pillars over tier; push pillars down through tier. Place 12-inch board on separator; top with 11-inch tier. Center 7-inch separator with pillars over tier (keep in mind placement of pillars in relation to tier below); push into tier. Set 8-inch board on separator; top with 7-inch tier. Arrange 7 quarts strawberries on tops of all tiers; serve sliced cake with remaining 5 quarts berries.
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