In a small saucepan, combine sugar and ¼ cup water over high heat. Without stirring, bring to a boil. Continue to cook without stirring until mixture turns a deep amber color and a digital thermometer registers 355° to 360°, 15 to 18 minutes. Remove from heat.
Carefully whisk in cream. Whisk in butter and salt until butter melts and is incorporated. Stir in vanilla extract. Transfer caramel sauce to a heatproof container and let cool completely. Store in an airtight container at room temperature and serve within 5 days.
Recipe by TeaTime Magazine at https://teatimemagazine.com/derby-blondies/