Line a baking sheet or plate with parchment paper. Drop dollops of Nutella onto parchment, approximately 1/2 teaspoon worth each (you can also use a piping bag or a plastic bag with the corner cut). Freeze for at least 30 minutes or until firm.Preheat oven to 300 degrees F. Stack two matching, heavyweight, light to medium colored cookie sheets one inside the other (stacking two cookie sheets together keeps the bottoms of the cookies from getting too brown). Line with parchment paper.In a large bowl, whisk together hazelnut flour, sugar and salt until evenly incorporated.In a mixing bowl or the bowl of a stand mixer fitted with the whisk attachment, whisk egg whites and lemon juice until they hold soft peaks.Add beaten egg whites and almond extract to dry ingredients and stir until mixture forms a soft, sticky dough, kneading with your hands if necessary. No need to be gentle here, we’re not making macarons. ;)Lightly dust your hands with powdered sugar. Use a small cookie scoop to portion dough into 1-inch balls. Split each ball in half, and flatten one piece somewhat on your palm. Place one frozen dollop of Nutella on top of flattened piece, then top with remaining half of dough, forming it over and around the Nutella and sealing up the sides.Roll into a smooth ball, then roll in powdered sugar. Arrange on parchment or silicon-lined baking sheets, leaving 1 inch of space between cookies. Repeat with remaining dough. If Nutella starts to soften, place the baking sheet back in the freezer for 5 or 10 minutes to firm it back up again.Bake for 30 to 35 minutes until tops are cracked and bottoms are just barely golden (if you are NOT using doubled cookie sheets your cookies will brown much quicker, and will likely only need 25 minutes, so watch them closely). If you prefer crunchier cookies you can give them an extra 5 minutes or so or until the tops begin to brown too. Remove from oven; let cool a few minutes, then transfer to wire racks to cool completely.Cookies will keep at room temperature in an airtight bag or container, for up to 5 days.