Preheat oven to 350°F (180°C) and line baking sheets with parchment paper.

In a mixing bowl, beat butter and sugar together until light and fluffy.

Add egg yolk, vanilla, and orange zest; mix until well combined.

In another bowl, whisk flour, hazelnut flour, and salt together. Gradually add to the wet ingredients and mix until a soft dough forms.

Roll dough into 1-inch balls and place on prepared baking sheets.

Make a small indent in each cookie using your thumb or the back of a spoon.

Bake for 8–9 minutes, then remove and re-press the indent if needed. Return to the oven for 6–8 more minutes, until edges are golden.

Spoon a small amount of jam into each indentation while warm.

Cool on a wire rack, then dust lightly with powdered sugar before serving.