Thanks for the recipe.

Soft sugar would be light brown sugar. It's also know as golden brown sugar.

Here's a link to some images.

If you cannot find it for purchase, you can make some yourself as

1 cup granulated cane sugar

1 Tablespoon unsulfured molasses

In a medium sized bowl, mix together the sugar and molasses.  It’s that easy.  There’s a part in this process where the molasses is super gunky and clumpy.  You’ll think to yourself:  Joy, you were wrong… this is coming out all wrong.  Don’t worry.  keep mixing it all together.  It will even itself out.   Work it until completely incorporated and no big molasses globs remain.  For dark brown sugar, add another tablespoon of molasses.  Use as you would in your favorite cake and cookie recipes.  Store in an airtight container or in a ziplock bag with the air pressed out.  Dang that’s easy! Source. http://www.joythebaker.com/blog/2010/08/how-to-make-brown-sugar/