The Engineer's Diet: eat all the ice cream you want! The energy your body uses to warm up the ice cream is greater than what is contained in the desert. It takes 1 calorie to heat 1 gram 1 degree. Since ice cream is frozen and your body is about 98 degrees and there are lots of grams in an average serving it takes a tremendous number of calories to get the desert to body temp. Hahaha - stop me here before I get carried away and break out more numbers! That is an old joke. In reality eating too much of anything, especially something containing both sugar and fat, is how to gain weight.
For the real engineer's diet check out:
http://www.carlwohlforth.com/diet/diet.htm. It has BMI and BMR calculators so you can estimate you Body Mass Index and Basel Metabolism Rate. In a nutshell you calculate your calories burned and calories consumed. Change your diet and/or exercise levels so you burn more then you eat and you will lose weight. Find permanent lifestyle changes you can live with, or better yet enjoy, and the weight loss will be permanent too.