The bar of chocolate experiment is a bit dodgy, see school teachers blog, in particular these comments.
To measure the speed of light, you measure the distance between melting "hotspots" in a chocolate bar in a microwave oven. You then convert that to the speed of the microwaves (which is the same as the speed of light), given the equation: the speed of any wave is the frequency times the wavelength.
It is difficult to identify the hotspots in the microwave, and to accurately measure the distance between them.
Also it's pretty difficult to understand or explain what exactly the hotspots are in the oven, why the waves are not moving. You need to understand interference and standing waves.
A bit like "real" science.