Cream together the butter or margarine
and sugar until light and fluffy. Blend in the egg yolk, then stir in
the flour and nuts to make a fairly stiff mixture. If it is too stiff,
add a little milk; if it is runny, stir in a little more flour. Spoon
the dough into a greased 30 x 20 cm/12 x 8 in Swiss roll tin (jelly
roll pan). Beat the egg white until frothy and spread over the
mixture. Bake in a preheated oven at 180°C/ 350°F/gas mark 4 for 30
minutes until golden. Leave to cool, then cut into bars.