please tell me ..
How can I make biscuit in AE and then break it to two parts and have some small pieces fall down ?
any help would be so much appreciated ..
Shoot it as a practical effect with a real biscuit on a chroma key background, then place it over the appropriate background using Keylight in After Effects. That is by far the easiest and fastest way to do it. There is a bonus: it looks absolutely real because it IS real.
Strictly inside AE you have two options. Shatter using custom shatter maps and masking + particle effects. Neither would be as conviceing as Dave's solution.
If you have access to a 3D app (blender is free) you can do it there.
Here's a 2 minute project in AE with a stock image:
with a shot of the broken buscuit top animated to fit the split it might be kind of convincing.
Here's the CS6 AEP project file for you to play with. Take a look at all of the elements to see how this worked by selecting all layers and then pressing the u key twice to see everything I changed.