Start with Soft Black on the vanity and cabinets. It is a deep, slightly warm black, so it reads rich and grounded instead of harsh, and it gives a small bathroom a real anchor to build around. Against tile and mirrors, it looks tailored and a little dramatic, which is exactly what makes a black-and-white bath feel intentional rather than plain.
Keep the walls in Pure White so the room stays bright and the black cabinets get room to breathe. The clean, soft white bounces light around and makes the space feel larger, while a touch of Light Oak in warm wood, a stool, a frame, or open shelving, softens all that contrast and keeps things from feeling cold. Black goes low on the cabinets, white wraps the walls, and the oak shows up in the small wood pieces you can move and add over time.