Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9x13 inch pan. In a saucepan, combine dates, boiling water and baking soda. Let stand while mixing the batter.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour, baking powder and chopped walnuts. Stir in the date mixture. Pour batter into prepared pan.
Bake in the preheated oven for 50 to 60 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.