Preheat oven to 375F. Beat butter, sugars, vanilla & water till creamy. Sift flour, baking powder and salt. Add to creamed mixture. Stir in nuts. Spread batter in 8" square pan. Sprinkle with chocolate. Bake in 375F oven for 1 minute. Remove pan. Run knife through top for marbled effect, bake for 15 to 20 minutes or till cake tested inserted inside comes out clean.