Preheat oven to 180 degrees C. Grease and flour a 20cm square cake tin.
In a large bowl mix the flour, sugar, bicarbonate of soda and salt. Make a well in the centre and add the egg, vanilla, mandarin and 1/2 cup chopped nuts. Mix well.
Pour batter into a 20cm square cake tin. Bake in the preheated oven for 30 to 35 minutes or until a toothpick inserted into the cake comes out clean.
While the cake is baking prepare the sauce. In a medium saucepan combine brown sugar, milk and butter. Bring to a boil and stir in coconut and 1/2 cup walnuts. While still hot pour over cake as soon as it comes out of the oven.