Clean a 1-pint glass jar and lid with hot, soapy water and let air-dry.
Add the heavy cream and cultured buttermilk to the jar and stir until evenly combined.
Loosely cover (lid set on but not tightened) and let stand at room temperature (70–75°F/21–24°C) until thickened to a pourable, spoon-coating consistency and pleasantly tangy, 12–24 hours.
Stir in the fresh lime juice and fine salt until smooth and glossy.
Seal and refrigerate until fully set and chilled, at least 4 hours before using.
Store refrigerated up to 10–14 days; stir before each use.