Preheat the oven to 350°. In a pie plate, toast the hazelnuts for about 12 minutes, or until fragrant and the skins have blistered. Transfer the hazelnuts to a kitchen towel and rub them together to remove the skins. Let the hazelnuts cool completely, then transfer them to a food processor and finely grind. Transfer to a bowl. Finely grind the almonds and add them to the hazelnuts. Using a wooden spoon, stir in the confectioner's sugar, then stir in the orange zest, the milk and the melted butter.
In a clean bowl, using an electric mixer, beat the egg whites at medium speed until soft peaks form. Gradually add the granulated sugar and beat at high speed until the whites are stiff and glossy. Sift 1/2 cup of the flour over the nut mixture and stir it in. Fold in half of the whites with a rubber spatula. Sift and fold in 1/2 cup of the flour, then fold in the rest of the egg whites.
Increase the oven temperature to 375°. Brush 2 large cookie sheets with melted butter and dust with flour, tapping off any excess. Drop scant teaspoons of the batter 1 1/2 inches apart on the prepared sheets. Bake the cookies on the upper and middle racks of the oven for 12 to 14 minutes, or until browned at the edges, shifting the sheets from top to bottom and back to front halfway through. Immediately transfer the cookies to wire racks to cool. Repeat with the remaining batter. Wipe the cookie sheets clean and let them cool completely. Butter and flour the sheets before reusing.
Melt the chocolate in a microwave oven, about 1 minute. Stir in the Nutella. Line 2 baking sheets with wax paper. For each sandwich, drop a scant 1/2 teaspoon of the chocolate filling on the bottom of a cookie; cover with another cookie and press gently so the filling spreads to the edge. Repeat with the remaining cookies and filling.Transfer the sandwich cookies to the prepared baking sheets and refrigerate until set, about 10 minutes.