In a medium bowl, whisk together the flour, icing sugar and salt. Set aside.

In a stand mixer fitted with the paddle attachment or in a large bowl using a hand mixer, beat the butter and sugar together on high speed until smooth and creamy, about 2 minutes. Add in the egg yolks, beat until combined. Stir in the vanilla. Slowly add the flour mixture, beating constantly, until all the flour is incorporated.

Chill the dough for 1 hour in the fridge.

Preheat your oven to 325 °F. Line two large baking sheets with parchment paper.

If you are using nuts, in a medium bowl, beat the egg whites until frothy and set aside.

Remove the dough from the fridge.

Roll a Tablespoonful of dough into a ball, then dip into the lightly beaten egg whites and then roll in the nuts.

Place on the baking sheet two inches apart. Press an indent into the top of each cookie. Fill the indent with your chosen jam.

Bake for 15-18 minutes, until the cookies are golden brown and the nuts on the outside have toasted and are browned.

Remove from the oven and move to wire cooling racks and let cool completely.

Store in a lidded container in layers, with parchment or waxed paper between each layer at room temperature for up to 5 days. Freeze for up to 3 months.