In a large bowl, sift flour, cocoa powder and salt. Set aside.
Cream butter and powdered sugar in a bowl of a standing mixer until light and fluffy. Reduce the speed to low and beat in the flour mixture until just incorporated.
Gather dough and form a dish. Cover with plastic wrap and refrigerate for about 30 minutes to chill.
Preheat oven to 350F. Grease 12 mini-tart pans a regular-sized muffin pan.
On a lightly floured surface, roll out the dough to 1/4 inch thickness. Cut into 5-inch rounds. Line the prepared pans with the dough circles. Use a small kitchen knife to trim the excess and to score the tops of the tartlets.
Place in the refrigerator for about 15 minutes before baking for 10 to 15 minutes or until lightly golden.
Remove from oven and let the tart shells cool a few minutes before inverting and transferring them to a wire rack to cool completely.
To prepare filling, in the bowl of a standing mixer fitted with the whisk attachment, beat the cream until it holds firm peaks.
Fold in Nutella to combine.
Spoon the Nutella mixture into the cooled shortbread tartlet crusts, and place them in the refrigerator to chill for at least an hour.
Sprinkle sea salt flakes on top of the tartlets before serving.