In this Video I explain how to make a slow burning firework putty using Marshmallows and an oxidizer (Sodium Nitrate in my case...) Potassium Nitrate can also be used but the ratio is 3 parts oxidizer to 2 parts sugar, instead of 2 parts oxidizer to one part sugar like Sodium Nitrate...
What you'll need for this is an oxidizer like Sodium or Potassium Nitrate, Marshmallows and a source of ignition.
I list the formula in the video in case you wish to scale it up or down, which I'll also list here, but note this is the one for sodium Nitrate not Potassium Nitrate...
Sugar content x 2 = NaNO3 Amount or Marshmallow (Weight)/2 = Sugar Content (Roughly) x 2 = NaNO3 (Sodium Nitrate) Amount (Divide by 1.875 instead of 2 if you wish to be more precise) or to keep things simple, Marshmallow (Weight) = NaNO3 Needed (Roughly...)
Sodium Nitrate from Ammonium Nitrate video (http://youtu.be/AlN5uMo9Cfc)
NightHawkInLight's Video (https://www.youtube.com/watch?v=0VPIoRVt30U)
Remember, though I find this fairly safe if done correctly, I am still not responsible for any damages that might occur if you replicate this experiment, and always make sure to have fire safety equipment nearby... And have fun!!