Mix the soft, room temperature butter with the sugar until creamy, about 10 minutes.
Add the egg yolks and continue mixing until smooth.
Turn off the mixer.
Mix the flour with the ground walnuts, salt, and almond flour.
Put the dry ingredients on top of the buttercream and incorporate with a spoon.
If you find the mixture too soft, can add another tablespoon or two of flour.
If the dough is too stiff, add 2-3 tablespoons of milk.
Wrap the dough in cling film and refrigerate for at least an hour.
Divide the dough into 2 pieces. Roll one piece out into a 0.3cm (0.12 inch) thin sheet.
Chill the other piece of dough again.
Cut out round cookie shapes and place them on a baking tray lined with baking paper.
Leave a little space between them.
Refrigerate the tray of cookies.
In the meantime, preheat the oven to 180 C degrees (356 Fahrenheit).
Cut the rest of the dough into rounds and chill them too.
When the oven is hot, put in the first tray of cookies.
Bake the cookies for 12-13 minutes, until lightly browned around the edges. Remove and place on a cooking rack to cool.
Continue with the remaining cookies.