Toffee Bars

1/2 cup sweet cream butter 1/2 cup brown sugar 1 egg yolk 1/4 cup all purpose flour 1/4 cup rolled oats 3 oz chocolate 1 oz sweet cream butter 1/3 to 1/2 cup chopped walnuts (alternative: slivered almonds -OR- chopped pecans -OR- chopped hazelnuts)
 * Ingredients:
 * Topping:

Method:
 * Grease a roll tin. Preheat oven to 375 F.
 * Beat butter, sugar and egg yolk together until smooth.
 * Add flour and oats. Stir until mixed.
 * Pour into tin, press down, tap tin a few times on a hard surface to get rid of any air bubbles, and then bake for 15 to 20 minutes until golden brown. Allow to cool slightly.
 * Melt chocolate in a double boiler until smooth, then add butter, whisk until smooth and glistening. Pour over bars and sprinkle chopped nuts over top.
 * Cut the bars into portions when the mixture is still warm, and then allow to cool completely.