You only need normal white sugar and molasses.
Molasses is what remains after sugar has been extracted from sugar cane. There are different grades of molasses, the thickest and most concentrated one called blackstrap molasses. I've never seen any other quality, so I presume this is the only one available here. Molasses has a very high content of minerals and vitamins making your brown sugar even a healthy thing.
In Germany you'll get molasses (= Molasse) from the Reformhaus.
Most recipes state just to combine plain white sugar with a bit of molasses with a fork. The first time I tried that and the result was good with just a few small lumps of molasses in it.
The second time I made brown sugar I couldn't get rid of the lumps. The third trial also had too much lumps. But then I remembered someone saying to reheat in a microwave shop bought brown sugar that had hardened. Obviously heat melts away these lumps.
My microwave currently is broken, so I tried a hot water bath.
I used around 750 g ( = 3 1/3 cups) white sugar and put it into a plastic bowl. The molasses I took out of the tin with the fork I used for mixing later. The molasses is fairly thick and stiff and this way you don't need an extra spoon from which to scrap off the sticky stuff. The amount I used roughly equals one tablespoon.
Update 12.Juli 2011: Bettina from Woolly Bits has a great tip: she uses a spoon lightly covered with oil, so the molasses easily runs off the spoon. Thank you, Bettina!
Adding more molasses gives you a darker brown sugar.
In a large bowl thoroughly combine sugar and molasses with a fork. You will still have some lumps. Fill another bowl, slightly larger than the first one, with some boiling water. Put in the sugar bowl and go on stiring until sugar and molasses are well blended and most lumps have disappeared. Take care not to spill water into the sugar bowl!
Take the sugar bowl out of the water, let the mixture cool down and dampen off, mixing through it every now and then. Then fill into an airtight jar or container.
Voilà! There's your brown sugar!
If you want to use brown sugar in cakes or in desserts that are going to be mixed with a handmixer or food processor, you can simply add white sugar and molasses from the tin.
Molasses gives a somewhat silky texture to food. If you add some brown sugar e.g. to a natural low fat yogurt, it will make this very creamy without adding any fat.
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