Why can you not bake with NutraSweet?

Answer 1

Nutra Sweet (one brand name for the chemical sweetener Aspartame) is sensitive to heat, and does not retain sweetness (in a reliable fashion) when subjected to higher temperatures. Aspartame also lacks sugar's shelf life, which why the "Sell by" dates on diet sodas are so different from the ones on regular, sugared sodas. Time and heat significantly fade the sweetness of aspartame.
What this means is that its uses are limited to recipes that do not require extended heating times. Unfortunately, that's pretty much what baking IS.
Using real sugar significantly impacts the end result when used in home baked foods. Moisture content, texture, color, all of these are affected by the use of sugar and how it reacts in combination with the eggs, fats and liquids found in baked goods recipes.
The manufacturers of aspartame and other sugar substitutes know this and continue to try to work with and around the problem, often combining their product with real sugar in various percentages. I've tried them all, and I'm sad to say - I have noticed little improvement in the dry, off-tasting results of artificially sweetened baked goods.
Best bet: Moderation. Smaller portions. Desserts sweetened with fruit (stop groaning, they're out there and they're good). Or - stick with diet desserts that don't require heating - Gelatin, instant pudding, smoothies, etc. Good luck!