In a large bowl, combine flour, baking powder, cinnamon, salt and hazelnut flour.

In a separate large bowl, or bowl of an electric mixer, fitted with the paddle attachment, combine butter and sugar until light and fluffy. Medium-high speed for approximately 2-3 minutes. Add egg yolks, vanilla and lemon zest and beat to combine making sure to use a spatula to clean edges of bowl.

With the mixer on low speed, add the flour - hazelnut mixture and blend until just combined, 15 - 20 seconds.

Turn out the dough onto a lightly floured work surface, divide in half and shape into flattened discs. Wrap in plastic and refrigerate for 1 hour or overnight.

When ready to bake, remove 1 disc from refrigerator and let stand for 5-10 minutes or until lightly softened. This keeps dough from cracking.

One a large piece of parchment paper, lightly dust with flour and roll to approximately ⅛" thick. To help prevent sticking, you may need to lightly flour your rolling pin and under the double as you work. Once rolled, transfer parchment with rolled dough onto a cookie sheet and refrigerate for 15-20 minutes or until firm. Repeat with second disc.

Preheat oven to 325 °F

Line two baking sheets with parchment. Remove one sheet of parchment with your rolled dough from the refrigerator and place on your work surface. Working quickly so dough stays firm, using a 3" heart shaped cookie cutter, cut out as many hearts as you can from your rolled dough. Using a 1.5" - 2" heart shaped cookie cutter, cut a smaller heart out of the center of half your 3" hearts. Using a spatula, place approximately 1.5" apart on your cookie sheets.

Bake for 12 - 15 minutes turning your sheets half way through. They are done with the hearts have turned a nice golden brown all the way around. Once done, transfer to a cooling rack until completely cool.

Once cool, spread the full heart pieces with approximately ½ tbsp of jam or enough to cover in a light layer. Dust the open heart sizes with confectioners' sugar.

When ready to serve, place open heart pieces onto jam covered full heart pieces.