Afghan biscuits

Dough

• 200 g unsalted butter, softened and cut into pieces
• 100 g brown sugar
• 180 g plain flour
• 30 g cocoa powder
• 1 tsp natural vanilla extract
• 60 g cornflakes

Chocolate icing

• 20 g unsalted butter
• 200 g icing sugar
• 20 g cocoa powder
• 2 - 3 tbsp water
• walnuts, for decorating