1. Sprinkle baking soda over chopped dates in a bowl.
2. Cover with boiling water and cool for 1 hour.
3. Prepare toffee by heating heavy cream in a saucepan and set aside.
4. Heat 1/2 cup butter or margarine and brown sugar in a medium saucepan to a boil and cook for 2 minutes while stirring.
5. Remove from heat, stir in cream and set aside.
6. Heat oven to 350 degrees.
7. Grease a 13x9-inch baking pan.
8. Beat 1/2 cup butter and sugar in a mixer bowl until fluffy.
9. Add eggs and beat 1 minute.
10. Alternately beat in self-rising flour and date mixture, beginning and ending with flour.
11. Pour into prepared pan.
12. Bake for 45 minutes.
13. Cool in pan for 1 hour.
14. Pour toffee over cake in pan.