Prepare a double boiler: Bring water to a gentle simmer in a medium saucepan (1-2 inches of water). Place a heatproof bowl over it, ensuring it doesn't touch the water.

Whisk egg yolks, sugar, and salt in the heatproof bowl until combined.

Slowly pour in the hot coffee while whisking constantly to temper the eggs and prevent curdling.

Stir the mixture frequently for 10-15 minutes over the simmering water until it thickens slightly.

Remove the bowl from the heat and stir in the heavy whipping cream until fully incorporated.

Let the sauce cool slightly before serving; it will thicken further as it cools.