Hazel would be perfect for a form like that you could get 2 knife's from out 1 small stick thus avoiding the pith centre. It carves real nice, even, cheesey texture, looks well when its dries (sort of like ivory) To cut cross grain (which is what you are doing on this handle end) with any woodwork tool saw/chisle/plane/axe/knife etc they MUST be real sharp to get clean cuts that dont tear out wood fibres and leave small holes. To do the end of that knife handle I'd do lots of little slicing cut's, gradually increasing the andle of entry until by the time I'm slicing at the centre of the "dome" I'm at 90 degree's to the axis of the knife. Lots of tiny cuts. If you try to take big cuts the knife will dig in possibly if its blunt and it could be a messy scenario.
I always think a butter knife should have a blade that is more or less parallell, with a slight curve along it, with a semi circular end to it, its nigh on impossible to get butter out the pot and spread it with these modern sloyd type designer pointed "eating knife's" with a sharpish point to them
