Platinum (element), thin sheet of metal
鉑 centers on platinum as a precious metal, and by extension a thin sheet of metal, likely from the idea of a flattened, refined form of the element.
鉑 combines 金 (metal) with 白 (white), likely referring to platinum as a white metal. The thin sheet meaning may derive from the idea of a flattened, refined form of the metal.
The left side 金 is metal, and the right side 白 is white. Picture a shiny white metal like platinum being hammered into a thin sheet.
For ハク, imagine a thin sheet of platinum being hacked off a larger block: hack -> ハク, and the white metal is cut into a thin piece.