You can't bring your own cake to a restaurant, dummies.

I mean, you can. But don't.

Sometimes, I think the general public understands the concept: Just as you shouldn't bring your own drill to the dentist or sing your own songs at a Broadway musical, you should expect to eat (and pay for) the food on the menu when you patronize a restaurant. But restaurant owners and workers assure me that no, people don't get it.

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So here's a reminder of why this is wrong. It's very simple. Restaurants are small businesses that operate on a slim profit margin. If you want them to exist, it's counterintuitive to bring your own cake.

I'm not talking about Kelsey's or Earls or Applebee's, which are corporations that sell food. I'm talking about actual restaurants. The majority of Canada's independent restaurants are in or near urban areas where real estate is at a premium. Rent is just one cost among many. To recoup those costs and hopefully turn a profit, restaurant owners buy food, prepare it and present it in a comfortable environment, for more than they paid themselves. So while bringing your own cake incurs no food cost to the restaurant, it's still asking to use their rent, insurance, utilities, labour and taxes for free.

This seems obvious to me. But apparently, when told not to bring their own cakes, people often do anyway. Even to dessert restaurants.

"This was an everyday occurrence," says Kelly Kimel, owner of MoRoCo Chocolat.

Now retail only, MoRoCo was originally a dessert restaurant in Yorkville, a plush, velvet-lined room located on the city's most expensive commercial real estate. It served all sorts of cakes and desserts, including vegan and gluten-free options, but despite their endless attempts to accommodate customers, even a business centred on desserts had to constantly explain to diners why they couldn't bring their own.

"We had written contracts stating 'no outside food' and they would still walk in and demand to have their special cake served," Kimel says. "Most of it was customers trying to save a few dollars."

When we dine out, we are renting space, and time is a commodity. A large group staying for dessert will take up an extra half hour, maybe a whole hour. With typical dinner operating hours being from 6 p.m. to 10 p.m., that's big chunk of time to cede without revenue.

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It may seem as if booking a party of 10 or 20 is buying in bulk, deserving a discount. But restaurant margins are notoriously slim. And yes, birthdays are special: to you. To a restaurant that serves hundreds or thousands of customers a year, every day is a potential birthday.

"We never say no to a guest," says Ed Ho, owner of Globe Bistro in Toronto, who says customers asking to bring their own birthday cake is a regular occurence. "We try to manage the situation on a case-by-case basis. We also generally buy dessert for the birthday person." For some restaurants, taking a loss on a group's dessert may be a key to maintaining a regular client.

In some circumstances, it might make sense to bring your own cake: If the restaurant, pub, water park or sex dungeon where you're hosting a celebration does not make or provide their own suitable desserts, or if your birthday boy or girl has a severe allergy that can't be accommodated by the kitchen. So long as you ask and the owner or manager says yes, it's kosher.

You'll probably be charged a small "plating" or "cakeage" fee, maybe $3 or $5 a person: Regardless of who made the cake, it takes the same amount of time for it to be sliced and plated, for servers to present and clear and for dishwashers to clean up afterward. It's not outlandish. ("Always check the condition of the cake the moment it is delivered to your staff, with the guest present," Arthur Bond, a server at Cava, advises restaurateurs. "The last thing you need is to be accused of ruining a cake that was actually damaged in transit by the guest or cake company.")

A few weeks ago, I had dinner at Momofuku Noodle Bar in Toronto, which is in a building housing multiple Momofuku businesses, including an outpost of its Milk Bar bakery. The friend I was with told me a story: A while back, he'd attended a birthday dinner upstairs at Momofuku Daisho. The dinner host had called ahead and been told no, please don't bring your own cake, but had brought one anyway. The result was an unpleasant standoff with staff and, ultimately, the guests had to carry the offending cake back home whole.

I called Daisho and asked if I could bring my own cake to a party. The reservationist explained they did not allow outside food, but talked me through options for an in-house cake that would work out to about $5.50 a person. Name me another nice restaurant that'll serve dessert for that price, especially one imagined by a pastry chef with two cookbooks.

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But the issue was likely never money. The request to bring your own cake is rarely about lack of options, and more often because people want to have their cheap supermarket cake served in a nice room where someone else cleans up afterward.

The truth is, if there's cake on the menu and you still want to bring your own, you're being selfish. Happy birthday.