When making the donut mixture, mix the wet and dry ingredients together only until combined, do not over mix. Over mixing the cake mixture will result in a tougher more gummy cake donut, not a light and fluffy one.
Sift the icing (powdered) sugar to remove any lumps before mixing with the liquids to make the glaze. This will give you a much smoother glaze to dip the donuts into.
A small whisk or fork is best when mixing the glaze together, to ensure all the ingredients are well combined, smooth and the glaze is lump free.
I would recommend using a gel food colour in this recipe. Gel food colours give a much stronger colour than liquid colours do, and they do not affect the flavour or texture as much. This is important so you can maintain the consistency of the glaze for donuts.
Add extra milk a few drops at a time to achieve the glaze consistency you prefer. I like my glaze quite thick, but do add more milk to make it thinner if this is your preference.
You do not need very much cocoa speckle paint for these donuts. Lightly dip the brush in once made and flick over from a height to get a lighter speckled effect. If you want larger flecks, flick nearer the donuts or try adding a little more cocoa paint to the brush.