a 6-inch and 8-inch cake board; 3.5-inch starfish candy mold; 4-inch seashell candy mold; 2-inch seashell candy mold; 2.5-inch sand dollar candy mold; a small, soft-bristle artist brush; printable sandcastle doors and windows template
Position the racks in the upper and lower thirds of the oven and preheat to 350 degrees F.
For the cakes: Coat two 8-inch round cake pans and two 6-inch round cake pans with nonstick baking spray with flour.
Whisk together the flour, baking powder, cinnamon and salt in a large bowl and set aside.
Combine the butter, brown sugar, vegetable oil and vanilla in the bowl of an electric mixer fitted with the paddle attachment. Beat until fluffy and light, about 5 minutes. Add the eggs one at a time, beating well after each addition.
Add half the dry ingredients to the butter-sugar mixture and mix on low speed until just incorporated. Add the buttermilk and mix again until combined. Add the remaining flour mixture and mix until just combined. Scrape down the bowl with a rubber spatula, folding in any ingredients that didn't get incorporated. Divide the batter between the prepared pans, filling each no more than 3/4 full.
Bake until a toothpick inserted near the center of the cakes comes out clean, about 30 minutes for the 6-inch cakes and 40 to 45 minutes for the 8-inch cakes.
Cool in the pans for 5 minutes, then turn the cakes out onto wire racks to cool completely. If necessary, level the cakes with a serrated knife or cake leveler.
For the frosting: In the bowl of an electric mixer fitted with the whip attachment, combine the butter and confectioners' sugar. Beat on low speed, gradually increasing to high as the sugar is incorporated and the mixture is light and fluffy. Add the dulce de leche in three additions until thoroughly combined. Beat in milk or cream 1 tablespoon at a time until the mixture is a spreadable consistency, about 2 tablespoons. Beat in the fine sea salt.
Transfer 3/4 cup of the frosting to a piping bag and set aside. Cover the remaining frosting with a damp towel so it doesn't dry out.
For assembly: Place an 8-inch cake layer on an 8-inch cake board. Top with 1 cup caramel frosting, spreading evenly to the edges. Top with a second layer. Cover the entire cake with a layer of frosting, using a cake smoother or bench scraper to make the frosting as smooth and even as possible. Chill for 20 minutes or until the frosting is firm.
Pour half the graham cracker crumbs onto a rimmed baking sheet. Remove the chilled cake from the refrigerator and roll the sides in the crumbs. Cover the top of the cake evenly with crumbs, gently patting them into the frosting. (The crumbs should stick naturally, but if they don't, brush the cake with a light coating of water using a pastry brush and roll again.)
Place a 6-inch cake layer on a 6-inch cake board and top with 1/2 cup caramel frosting. Repeat the frosting, chilling and crumb-coating process as before. Place the finished 6-inch tier on top of the 8-inch cake. Anchor the two cakes together using one long cake dowel, if desired. Refrigerate the cake while you prepare the remaining decorations. Reserve the pan of graham cracker crumbs to use later.
Make the spires and turrets by frosting the top inner edges of two cake cones with the caramel frosting. Nest a second cake cone inside each frosted one. Coat the outsides of the cones with frosting using a small offset spatula, then roll both cone stacks in the reserved graham cracker crumbs until completely coated.
Frost the exterior of two sugar cones with the caramel frosting and roll in the graham crumbs. Snip a small hole at the end of the piping bag and pipe a thick line of caramel frosting on the inside of each cake cone and top with the coated sugar cones so they point upward.
Trim away 1 inch from the bottom of one of the remaining sugar cones with a serrated knife using a gentle sawing motion. Cover the cone with caramel frosting and roll in the crumbs. Coat the remaining sugar cone with frosting and roll in the crumbs. Refrigerate all the decorated cones until firm, about 15 minutes.
Remove the cake from the refrigerator and position the two assembled cake cone towers on either side of the front of the cake. Anchor each in place using a dot of frosting. Arrange the two remaining decorated sugar cones on top of the cake. Pipe a mound of frosting under each cone to anchor it in place.
Put the brown sugar cubes in a large bowl and drizzle with honey. Toss with a fork to coat the cubes with honey, then toss with some of the remaining graham cracker crumbs. Arrange the coated cubes around the top edges of the cake tiers. Use a dot of piped frosting to anchor each cube in place.
Use the remaining frosting to cover the serving plate. Generously sprinkle the remaining graham cracker crumbs over the frosting to create a sandy beach appearance. (You may have leftover crumbs.) Refrigerate while you prepare the candy decorations.
Make the decorations: Combine the orange and red candy melting wafers in a microwave-safe bowl. Microwave in 30 second intervals, stirring well with a rubber spatula between each, until the candy is melted and smooth. Transfer the candy to a piping bag, snip the end and pipe into the starfish candy mold. Put the candy molds on separate small trays or baking sheets. Rap each pan on the countertop to remove air bubbles and freeze until set, about 20 minutes. Turn the candy out onto a work surface immediately after removing from the freezer.
Place the white candy melting wafers in a microwave-safe bowl. Microwave in 30 second intervals, stirring well with a rubber spatula between each, until the candy is melted and smooth. Transfer the candy to a piping bag, snip the end and pipe into the shell and sand dollar molds. Put the candy molds on separate small trays or baking sheets. Rap the pans on the countertop to remove air bubbles and freeze until set, about 20 minutes. Turn the candy out onto a work surface immediately after removing from the freezer. Allow all the candy decorations to come to room temperature.
Dip a dry art brush in the cocoa powder and tap off excess. Lightly brush across the white candy pieces to give them dimension. Arrange the candy shells and starfish around the cake tiers as desired.
To make the castle doors and windows, put the melted semisweet chocolate in a piping bag and snip a small hole at the end. Put the printed template on a small sheet pan. Cover the template with parchment paper and pipe chocolate onto the paper following the outlines of the shapes on the template. Use a toothpick to spread the chocolate into corners and fill any holes. Freeze the chocolate until firm, about 10 minutes, then turn the pieces over using a toothpick. Handle the pieces as little as possible to avoid melting the chocolate.
Pipe caramel frosting on the back of each chocolate piece and attach to the sandcastle.
Use a metal skewer or cocktail pick to gently tap a hole into the tip of each sugar cone. Insert the cupcake flags into the sugar cone spires.
Bring the cake to room temperature before serving.