Cool in pans 10 minutes before removing to a wire rack. For filling, melt butter and brown sugar in a saucepan over medium heat. In a small bowl, combine flour and salt with a small amount of milk; stir until smooth. Add the remaining milk gradually. Add egg yolk and mix well; stir into saucepan. Cook and stir over medium heat until very thick, about 10 minutes. Add vanilla and nuts. Cool. Spread between cake layers. Dust with confectioner's sugar. Chill. Store in the refrigerator.