Preheat the oven to 325°F. Line a 9-inch square baking pan with parchment. In the bowl of a food processor, cream together the butter and sugar until light and smooth, then pulse in the egg yolk. Pulse in the flours and salt to create a stiff dough.
Press the dough into the prepared baking pan and prick all over with a fork. Scatter the hazelnuts over the top, gently pressing them into the dough. Bake until golden brown, about 30 to 40 minutes. Transfer to a wire rack and allow to cool in the pan for several minutes before glazing and scoring into cookies.
Elderberry-Honey Glaze
⅓ cup elderberry syrup
2-4 tablespoons honey, or to taste
1 cup powdered sugar
In a small saucepan, whisk together the elderberry syrup, honey and powdered sugar and set over low heat. Continue stirring until the glaze becomes thick enough to coat the back of a spoon.
Allow the bars to cool slightly before glazing, then pour glaze over the shortbread and smooth to the edges with a spatula. Cut shortbread then serve.
