Use the freshest nuts you can find for this slice as old tannic walnuts will ruin the flavour.
Rolled Oat Base ¾ cup standard flour ¼ tsp baking soda ½ cup brown sugar 1 cup rolled oats 120g (4 oz) butter Chocolate Walnut Filling 3 Tbsp cocoa powder ¾ cup caster sugar 50g (1¾ oz) butter, melted 1 medium (size 6) free-range egg 1 cup standard flour ¼ tsp baking powder ¼ cup milk ½ tsp vanilla extract ½ cup chopped walnuts Icing 1½ cups icing sugar 1½ Tbsp cocoa powder 2-3 Tbsp hot water ¼ cup chopped walnuts
1 Preheat oven to 180°C (350°F). Sift flour and baking soda together into a bowl. Stir in brown sugar and rolled oats. Melt butter, pour into dry ingredients and mix together to a crumbly paste. Press into a greased tin 20cm x 30cm (8” x 12”). See notes for making a paper case in Recipe Notes below. Bake for 10 minutes.
Chocolate Walnut Filling
Blend cocoa powder and sugar together in a bowl and mix in melted butter. Cool mixture for 10 minutes, then mix in egg. Sift in flour and baking powder and fold everything together adding the milk and vanilla extract, and lastly, the walnuts. Spread over the base. Bake for about 25 minutes, until firm to the touch.
Once the slice is cool make the icing. Blend icing sugar and cocoa together in a bowl and beat in 2 tablespoons of hot water. This should make a stiffish but spreadable icing. Add a little more water to thin or a tad more icing sugar to thicken if necessary. Spread over the top of the slice. Sprinkle with walnuts. When icing is set, cut into bars or squares. Store airtight.
If your tin is too big, measure the size required on a piece of baking (parchment) paper, trim paper leaving a border all around, then fold in the sides and clip together with metal paper clips. There is no need to grease the paper.