With an electric mixer cream together the butter and brown sugar. Beat in the egg yolks followed by the flour.
Shape the dough into 1 1/4 inch balls. Roll the balls in the beaten egg whites. Then roll in the chopped walnuts. Place on a cookie sheet, lined with parchment paper. Press your thumb into the centre of each ball.
Bake in a 375F oven for 10-12 minutes. Cool on a wire rack.
Before you serve these cookies, fill the centre with jam and drizzle with icing sugar drizzle (1/4 cup of icing sugar mixed with a few drops of water to make a drizzling consistency)