Cream together the butter and sugar until creamy and completely combined.
Add in the egg yolks, beat until combined.Beat in the flour until completely mixed in.
Chill the dough for an hour in the fridge.
Preheat your oven to 325 °F.
Roll about 2 teaspoons of dough into a ball, then dip into the lightly beaten egg whites then roll in the walnuts, then place onto a parchment lined baking sheet. Repeat until the dough is all used.
When the cookies are all on the parchment lined cookie sheet, press a well in the center of each ball prior to baking using your thumb, of course!
Bake in the preheated oven for 10 minutes, then open the oven and re-press the wells using the handle of a wooden spoon to gently indent the cookies again. While the cookies are still on cookie sheets fill the wells with the raspberry jam. Return to the oven.
Continue baking another 8 minutes until golden.
Remove from the oven and move to wire cooling racks and let cool completely on the sheets.