In a medium saucepan, combine cream, sugar, butter, espresso powder, and salt.
Bring mixture to a low boil over medium heat, and cook, stirring constantly, until thickened and mixture coats the back of a wooden spoon, about 20 minutes.
Notes
Time can vary greatly depending on your stove . It only took 11 minutes for me. Let caramel cool until just warm (hot caramel will melt panna cotta).