Preheat oven to 350°F. Place 6 mini tart shells on a baking sheet and set aside.
In a large bowl, combine crust ingredients. Mix thoroughly to combine, then press mixture into tart shells (I used a half cup measurer to make a perfect indentation for my crusts!).
Bake for 10-12 minutes, then allow to cool completely.
To make the salted caramel, place a small saucepan over low heat. Add in sugar and cook, stirring occasionally, until the sugar melts completely, about 8-10 minutes.
As soon as your sugar is fully melted, remove from heat and add in butter. Whisk carefully until fully incorporated, then add in salt and 3 tablespoons heavy cream. Allow to cool until it’s warm but not hot.
Spoon caramel into the tarts until they’re just shy of full. Sprinkle in all but 2 tablespoons of the hazelnuts. Place in freezer to chill for 10 minutes.
Chop those remaining 2 tablespoons of nuts so they’re finely chopped. Set aside.
To make the ganache, heat 1/3 cup heavy cream in a small saucepan over low heat until scalding. Remove from heat, add in dark chocolate. and let sit for 5 minutes. After, whisk to combine. Mixture should be totally smooth and melted at this point.
Remove tarts from freezer and spoon ganache on top. Sprinkle with remaining hazelnuts and finish with flakey sea salt!