Preheat oven to 180 degrees C. Grease and flour a 23cm square pan. Combine the flour, bicarb soda and baking powder; set aside.
In a medium bowl cream together the sugar, butter and eggs until smooth. Add the flour mixture and beat until smooth. Finally stir in the sour cream and vanilla. In a separate bowl, combine the nuts, icing sugar and cinnamon.
Spread half of the batter into the pan. Sprinkle a layer of the nut mixture then spread the remaining batter and top with the rest of the nut mixture. Spread the melted butter over the top.
Bake for 1 hour in the preheated oven, until cake springs back to the touch.