Use a ruler to measure the distance between the centres of the two melted points. Defined as being 299792458 meters per second in vacuum you can take a crack at m. If you can figure out how fast microwaves travel your result should be roughly the same as the speed of light.
All electromagnetic waves travel at the same speed.
And since many of the suggested experiments also involve chocolate it will be done. You might already know that the speed of light is 299792458 meters per second but now you can confirm that number by playing kitchen scientist and melting chocolate. Multiply the distance between the spots on the chocolate bar by two. Microwaves and visible light are both electromagnetic waves so microwaves travel at the speed of light.