BetterEnd/utility_res/block/%name%_half_slab.json

{
        "parent": "block/slab",
        "textures": {
                "bottom": "betternether:block/%name%_planks",
                "top": "betternether:block/%name%_planks",
                "side": "betternether:block/%name%_planks"
        }
}