Pre-heat oven 180 C. Grease and line a lamington pan with baking paper.
In a medium bowl combine sugar, butter, egg and vanilla. Stir in sifted flour and oats and mix well. Press firmly into tin and smooth top. Bake in oven for 25-30 minutes or until it is a nice golden colour.
Combine dark chocolate, condensed milk and extra butter in a saucepan. Stir over a low heat until chocolate is melted, then remove from heat and stir in walnuts. When bake is cooked spoon chocolate mix onto the top and allow to set and cool completely, then slice.