Lightly grease 20 cm square baking pan. For cake, combine dates, boiling water and soda in bowl until well blended. In separate, larger bowl, cream butter and sugar until light and fluffy. Stir in vanilla. In third bowl sift together, flour, baking powder and salt. Add date mixture and flour mixture alternately in two batches to butter mixture. Stir in walnuts if using. Pour into prepared pan. Bake in preheated 180ºC oven 30 minutes. While cake is baking, make topping. In small saucepan, melt butter, brown sugar and cream; bring boil, stirring constantly. Remove from heat; stir in coconut. Spread over warm cake. Preheat broiler; broil cake until topping is bubbly, watching constantly to keep from burning. Cool; cut into squares.