It is probably going to happen to you. You are a healthy eater, you like yourself enough to do the things that your future self will be grateful you did. Heck, you LOVE your future self and are really looking forward to meeting them. For them you would do almost anything. They do not exist yet, but you are willing to do almost anything to make sure they have the best life possible. Good for you! They will thank you for it.
Oh, wait a minute, there is a chocolate pie. It is right there for present self to enjoy. For YOU to enjoy. Future self does not need to know about it. You can just have a slice and never speak a word of it, to them, to anyone. It will be like it never happened. Except if it does happen it DID happen.
Saying no all the time can be tough. It is possible and there will be a time in the future when you are glad you did. But life is not necessarily all about the future. The present moment, now, is actually a lot more real than the future. There are a lot of things to be said about getting the most out of each moment and sometimes that means saying yes to something that is a small speed bump in your journey.
You should give yourself permission to say yes to the things you like but that hurt your progress so long as you are not addicted to them and so long as they do not trigger an all or nothing type of event. Alcoholics are best to always say no to that first drink. X smokers are always better served to never take another puff. People on a massive weight loss journey are best to stick with the program until they achieve their predetermined celebration milestones. If this is not you, if you are a healthy eater who avoids sugar and baked desserts and do not have any celebrations lined-up, saying yes to the chocolate pie occasionally is not a big deal and, if done with planning, can have a very small impact on your progress.
Pay your bill BEFORE you eat. If you are going to enjoy an occasional 750 calorie dessert, create the 750 calorie deficit before you get stuck in.
If you are going to eat it on Sunday, in the 4 or 5 days before, eat 125 calories less than what you normally would or burn off an extra 125 calories with exercise. So long as doing this does not drop your calories below 1200-1400 on any of these days, it will be a wash and everything will work itself out by Tuesday or Wednesday of the following week.
Better than the lay away approach is to burn the calories as close to before the meal as possible. An intense work out primes the body to shuttle the calories into the cells that need them for recovery before moving them into fat cells for storage. If you have depleted 500 calories of muscle glycogen just before you eat a piece chocolate pie, many of the sugar calories you consume will be help replenish this energy. The sugar will spike your insulin which will also help with protein synthesis further aiding recovery and possibly help you gain some lean muscle. From time to time it is fine to look at food as molecules void of any context, provided you have done the work right before the use the molecules as you would if they were just individual ingredients.
Chocolate pie is NOT the same thing as coco, glucose, fructose, saturated fat, unsaturated fat, and water. The ingredients interact with each other to do things that the individual ingredients would not do on their own. Glucose and fat consumed together create a massive release of reward chemical that glucose or fat on their own do not. The whole is great than the sum of its parts. But occasionally and when done mindfully these treat experiences can promote a great sense of well-being with very little or no negative impact on future you. Just make sure you pay the bill before eating.