The “extra block” is simply a trick, or a geometric illusion. If you only count the blocks in the chocolate bar, you will find the same number before and after the experiment. The chocolate bar almost looks the same as it did before you took away the extra block.
However, if you measure its length, you’ll notice that it’s actually smaller than it was before. Every time the chocolate bar is cut and the pieces rearranged, a small amount of chocolate goes missing. This small amount is equal to the size of the “extra” chocolate block, but spread out across the width of the chocolate bar, so it is harder to notice.
That means that there’s less chocolate in the bar than before the experiment, and the extra block isn’t really extra at all.