These cookies are a cross between traditional Linzer Augen (called "eyes" for the cut out circles) and hazelnut shortbread. The addition of the cornstarch/potato starch creates a melt-in-the-mouth texture, so don't be tempted to leave it out.
To toast hazelnut meal (I use Bob's Red Mill brand), preheat your oven to 300ºF. Spread the meal evenly on a parchment lined baking sheet and place in the preheated oven. Every minute or so, stir it around to ensure even browning. It should only take a few minutes to become golden, and you'll know it's done by the toasted (not burnt!) smell. —Melanie K.
Mix butter and sugar until blended, either with a mixer or by hand. Do not cream mixture as you would chocolate chip cookies.
Add vanilla, cinnamon and salt, then mix in the toasted hazelnut flour. Mix in flour and starch until just combined. Do not overwork the dough!
Chill the dough, wrapped in plastic, for at least 20 minutes.
For sandwich cookies, roll out to 3mm (1/8″) thick. For regular shortbread, 4-5mm. Cut into desired shapes. For traditional Linzers, use a smaller cutter to make a “window” in the sandwich tops.
Place cutouts on a parchment lined baking tray and chill in the freezer for a few minutes. This helps the biscuits to keep their shape in the oven.
Bake until edges are lightly browned, 8-10 minutes for thinner cookies, 10-12 for thicker.
When cool, sift icing sugar over the tops with cutouts. Spread a teaspoon of jam on each of the remaining halves and sandwich the tops/bottoms together.