Clean leeks thoroughly between layers. Melt butter in large pot over medium; cook leeks 8-10 minutes, stirring occasionally, until very soft without browning.
Cook garlic 1 minute until fragrant.
Add potatoes, broth, thyme, and bay. Boil; reduce to medium-low.
Simmer uncovered 20-25 minutes until potatoes very fork-tender.
Remove thyme and bay; immersion-blend completely smooth. Alternatively, carefully blend in batches, taking care with hot liquid.
Add optional cream; season with salt and white pepper. Thin with extra broth if needed.
Serve hot with chives and optional bacon; also serve chilled if desired.