Peel plantains and cut into 1-inch rounds.
Heat oil to 325°F. Fry plantains 3–4 minutes until tender but not browned; drain.
Flatten with a tostonera or between parchment sheets.
Raise oil to 350–375°F. Fry a second time 2–3 minutes until golden and crisp; salt immediately.
Blend sauce ingredients until smooth, thinning with water as needed. Serve with hot tostones.