LeoNas is right in his comment, in printing this process is called Trapping. You can do it manually as LeoNas propose or automatically as Adobe indicates in this page:
If the document is in RGB mode, choose File > Document Color Mode > CMYK Color to convert it to CMYK Mode.
Select two or more objects.
Do one of the following:
To apply the command directly to the objects, choose Window > Pathfinder, and choose Trap from the panel menu.
To apply the command as an effect, choose Effect > Pathfinder >Trap. Select Preview if you want to preview the effect.
Set trap options, and click OK.
Anyway, I do not think that small gap will be visible when printing.
In vector illustration is better not to follow the traditional way of thinking when drawing:
A vector solution to your drawing would be selecting the whole chocolate piece > copy/paste in place > apply Unite from pathfinder > fill it with the darker color and send it back.
Added after the second image at the question: When drawing with a brush and filling it after, there's no possible way for these white lines to appear. I wonder about your drawing process: