I think when you're printing something that small slowing down the speed won't help you in terms of letting the layers cool. Basically you've got a hot extruder head pressing up against a small surface so no area of the surface really has a chance to cool no matter how slowly you're printing. Try printing two or three of the cubes in the same print--when the extruder is working on the other cubes the layer will have time to cool and solidify--then see if you still get the melty corners issue.