Basic Trope: Inedible items that look like real food are used for display purposes, and/or to advertise the real food.
Straight: A wedding cake displayed in a bridal magazine is little more than Styrofoam covered in fondant.
The actual cake on display at Alice and Bob's wedding is nothing but Styrofoam painted to look like it's iced.
The Styrofoam cake is displayed alongside various other fake foods.
A wedding cake displayed in a bridal magazine is a real cake, but on a smaller scale than the real thing.
A wedding cake displayed in a bridal magazine is a real cake iced with fondant. The real one being advertised would have buttercream icing, or could if the couple wished.
The picture in the magazine is only an advertisement for a bakery, a caterer with a pastry chef on the team, etc.; it's just not worth making an actual cake (which costs more) that no one's actually going to eat just to take a few pictures of it.
Cakes are fragile (particularly chocolate cakes and/or cakes frosted with buttercream frosting), and the hot lights of a photography studio could result in the cake melting and falling over, dyes running, etc.
Alice and Bob are having an outdoor wedding in summer or tropical weather. In order to save money, or to prevent a buttercream-frosted stacked-layer cake from falling over and/or guests coming down with food poisoning from leaving it in the heat, Alice has opted for a "rental cake": The display cake is mostly Styrofoam, with the exception of the bottom layer, which is cut. After the cake-cutting, the "cake" is wheeled back into the kitchen "to be served", and replaced with sheet cake in the specified flavor and style.
The bridal magazine features a picture of a real cake, made just for the occasion. Some of the cake was eaten, but most of it wound up in the trash after being photographed.
The pastry chef opted to have his/her work done for Alice's wedding displayed in the magazine.
The fake cake appears alongside other foodstuffs on display at the wedding, and Alice and Bob cut the cake together.
The cake in the magazine is indeed a real cake.
The appearance of the cake displayed at the wedding greatly differs from the cake shown in the bakery. However, it turns out that it's simply a different style of cake.
But they only cut through the bottom layer, the only part that was actual cake and not Styrofoam. The cake served to their guests is sheet cake, back in the kitchen.
But it is scaled down, and shot from such an angle that it looks like a full-sized cake, and/or any flaws have been airbrushed out.
Enforced: "We've used almost every imaginable joke on this show, except the classic Fake Food joke. But where could we use it?" "Here's an idea. Alice and Bob are getting married later this season, right? We could have a fake cake at their wedding."
Lampshaded: "That's no cake! It's just Styrofoam covered in fondant. Alice must have ordered a fake cake." "Well, duh! Would you rather die of food poisoning?"
Carrie's Cakes is taking out an advertisement in Bridezilla Magazine, to showcase her wedding cakes and hopefully get business from brides-to-be (and grooms-to-be)... But she doesn't want to spend all that time and money making a cake that no one's actually going to eat and will probably just wind up in the trash after the photoshoot.
The wedding is scheduled for April 1st, and Alice orders a fake cake as a prank.
Exploited: Alice and Bob's friend Charles finds out about Alice's "fake cake" prank, and switches the Styrofoam cake with a real one - thereby ruining the prank.
Carrie takes pictures of the finished cakes just before they are put on display, and keeps them in a portfolio, so that couples will look at them and see exactly what kind of work she can do. She submits a few pictures to the magazine to be used in the ad.
Alternatively, Carrie and her team make a completely real and edible cake (albeit on a smaller scale), and eat it after the photoshoot. That way, the photoshoot showcases what she can actually do and what the cakes actually look like, but the cake doesn't cost too much to make or go to waste after the photoshoot.
Alice and Bob have a cake on display during the reception that is 100% real cake. This cake is eventually cut and served. If the wedding is outdoors and food poisoning is a risk, they may omit the display part until just minutes before the cake-cutting, or store the cake in a refrigerated display cell.
The bakery or pastry chef relies on word-of-mouth advertising only.
Discussed: "This doesn't look like the one in the magazine. Are you sure it's real and not Styrofoam?"
Conversed: "Look closely. That cake is fake." "Of course it's fake! If they left it outside during Alice and Bob's wedding, people would come down with food poisoning."
Implied: The cake is shown in the window of a bakery when Alice orders it. The cake on display at Alice and Bob's (indoor) wedding has a visibly different texture.
Played For Laughs:
The cakes Carrie makes are exactly like the Styrofoam cakes in the magazine... Because Carrie is a Cordon Bleugh Chef and couldn't bake her way out of a paper bag!
Bob bites the fake cake and asks Alice, "What's wrong with this cake?"
Reconstructed: The pastry chef makes every effort to make the cakes he/she actually makes look as good as the ones in the magazine, and explains to customers that while he/she will put in a lot of work to make it look great, the "cake" illustrated in the magazine is made of Styrofoam and therefore does not look exactly like the real thing.
Untwisted: Alice orders a cake which is illustrated as Styrofoam in the magazine. She tells her guests how delicious the cake will be ... except she accidentally ordered a fake one.
Unparodied: Alice and Bob talk about the cake as if it were "The Emperor's New Cake", but it turns out it's the real thing after all.
Feel free to head back to Fake Food, but there's no real cake there either.