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

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