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 nuts. 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, spoon 1 tsp of Nutella in each cookie. Place 6-8 m&ms on the Nutella of each cookie.