The hardest thing about low-carb cooking is probably finding a suitable sugar-free substitution for sugar in your baking. There are a variety of options from which to choose:
Splenda, or sucralose, is the most popular and probably the easiest to work with. Because it’s made from sugar, it has a flavor very close to sugar, but it is 600 times sweeter. It has several drawbacks. Most important for low-carb diets, there are bulking agents added to the packets and the baking splenda, to keep it from clumping. As a result, there’s about 1 carb per packet. It can lose some of its sweetness with heat, which can effect baked goods. For that reason, it’s not suitable for making sugar-free chocolate since it causes the chocolate to seize up and become very bitter.
If you like the flavor of splenda, you can buy liquid sucralose online. It is carb-free and doesn’t lose its sweetness in baking. The one drawback is the lack of bulk, which can throw off the proportions of dry to wet in a recipe.
Equal, or aspartame, is an older artificial sweetener, which has garnered a lot of controversy over the years. There does seem to be some evidence that aspartame can lead low-carbers to go out of ketosis in about half of the people who use it. It loses some of its sweetening power when exposed to high temperatures, so it’s not great for baking.
Acesulfame potassium, or ace-k, is often used in conjunction with sucralose in sugar-free products. It is available in packet form under the name SweetOne. It is 200 times sweeter than sugar. It is stable when exposed to high temperatures, and combining it with sucralose results in more rounded sweetness, closer to sugar.
Stevia is an entirely natural, sugar-free, plant-derived sweetener. It can be bitter in large amounts, and has a very top-note sweetness. It is often better when combined with other sugar-free sweeteners, such as erythritol.
Sugar alcohols are derived from plants and are another great alternative to sugar. Erythritol is probably the best of the bunch, since it doesn’t cause the intestinal problems associated with xylitol, sorbitol, maltitol and isomalt. It is only 60% as sweet as sugar, and has a cooling feel in the mouth when dissolved in water which makes it seem minty. For that reason, you will want to mix it with another sweetener, such as stevia, to limit the amount used in a recipe. It can also have a drying effect on baked goods.
Xylitol, maltitol, isomalt and sorbitol are all sugar alcohols widely available commercially. They have a good sweetness rating, and you will often find them in combination in commercially-produced sugar-free candies, but they all will gas and stomach discomfort if you eat too much. They can also have some impact on blood sugar, and the amount varies from person to person.
The best results in baking come from the use of several sweeteners together. The resulting synergy produces in a rounded sweetness that is closer to sugar and needs less of each sweetener than would be needed with the use of any one sweetener.
One additional ingredient for your low-carb baking is polydextrose. It is derived from glucose, but cannot be digested. Because it is only 10% as sweet as sugar, it must be combined with other sweeteners. It gives the bulk and many of the properties of sugar to sugar-free baking, and can also be used as a substitution for fat and to add moisture to recipes that tend to be dry (such as those made with erythritol). It does come with a fairly high level of fiber and can cause digestive problems if you eat too much in one go. It can be used to create candies, syrups, puddings, ice cream and chewy cookies – basically, anything that relies on the thickening effect of sugar.
There is also a new sweetner on the market from the makers of Splenda called Nectresse. It's a good example of synergy at work - it's a combination of monk fruit (which is 200x sweeter than sugar and has no impact on blood sugar levels), erythritol, sugar and molasses. In fairness, it does have some carbs - the manufacturers are allowed to advertise it as calorie-free since it's less than 5 calories, but there is 1 carb per packet. Because it is so new on the market, it remains to be seen how well it will work in baking.