Sift flour, baking powder, ground cinnamon, and salt together. Process hazelnuts until finely ground. Then mix it into the flour mixture and set aside for later use.
Beat softened butter and granulated sugar until light and fluffy, about 3 minutes. Then add egg yolks and vanilla extract. Continue beating to combine.
Fold in the hazelnut-flour mixture. Divide dough in half, shape into flattened disks. Wrap each disk individually in plastic and refrigerate for at least 1 hour or overnight.
When you are ready, roll a disk of dough on a large piece of wax or parchment paper to about 1/8-inch thickness. Then transfer both paper and dough to a baking sheet. Freeze until firm.
Repeat with the remaining dough. Don't hesitate to stack one layer of parchment and dough on top of another.
HINT: To keep refrigerated dough from cracking when rolled, let it stand at room temperature until slightly softened.
To prevent dough from sticking to parchment or wax paper, dust paper slightly with flour before rolling. Add a little bit of flour on top of dough from time to time while rolling.
Line cookie sheets with parchment paper. Remove one sheet of dough and parchment from the freezer, and quickly cut into heart shapes with a 3-inch cookie cutter. Cut out center from half the shapes with a 2-inch cutter.
Transfer heart cookies to prepared cookie sheets, spacing about 1-inch apart. Freeze again for about 15 minutes until firm.
You can either bake the cutout centers or re-roll them to make larger linzer heart cookies.
Preheat oven to 325 degrees Fahrenheit just after you place the cutout cookies in the freezer the second time. Bake until linzer heart cookies are crisp and lightly golden all over, about 10-15 minutes. Be careful not to let them burned.
Cool these homemade cookies completely on wire racks, and store them in an airtight container before filling.
Just before serving, spread the flat side of the whole hearts with some jam. Sift confectioners’ sugar over the open hearts. For premium quality, fill and enjoy these homemade heart cookies on the same day.
