- Cook water, sugar, corn syrup, and salt to light-medium caramel color, then remove from heat and carefully whisk in the heavy cream and vanilla to stop the cooking process.
- Allow mixture to cool approximately 10 minutes before adding the butter and chocolate. Stir until incorporated.
- Mix ½ cup of each toasted nut into a bowl and evenly coat them with the caramel chocolate mixture. Portion filling into tart shells (smoothing out to the edges if needed), then top with some of the mixed nuts and lightly sprinkle with fleur de sel if desired.
- Allow the filled tarts to set for 20 minutes at room temperature before serving.