You may do it using a cloth simulation, a little bit of sculpting and shape keys.
Select your chocolate bar, apply the transformations to it (Ctrl+A-->Rot&Scale), go to the Physics header and press the Cloth button. Play with the settings in a Cloth panel.
Select the plane, apply the transformations to it as above and press the Collision button (remember to place the bar a little bit above it).
Select the bar and press Alt+A. Wait for the simulation to be finished, then go to the last frame (Shift+Right Arrow). Go to the Modifiers header and press the Apply as Shape Key button in the Cloth Modifier window. Now you have a new shape key in the mesh data header.
Now with a Cloth shape key selected enable the Sculpt Mode. Use the Clay brush to reduce the mesh distortion and the Grab brush to shape the melted chocolate bar.
Use the Smooth brush to make the edges smoother. Press I over the shape key value to keyframe it. Go to the next frame on the Timeline, change the shape key's value and keyframe it again.