Preheat the oven to 350 degrees F. Butter and flour two 9-inch-round cake pans or 20 muffin tins.
Place the dates in large saucepan with 3 1/2 cups cold water. Bring to boil, stirring a little to break up the dates. Then leave to simmer for 1 minute before removing from the heat. Stir in the baking soda (which will cause the mixture to bubble up).
Cream the butter and sugar together in a food mixer until pale and creamy. Add the eggs, one at a time, occasionally scraping down mixing bowl. Add the vanilla extract and then the flour and salt and mix briefly to give a lumpy dough.
Next, add the warm date mixture in two batches. Scrape down the sides of bowl in between mixing. The dough will now be quite watery but don't worry! Finally add the baking powder (this will bubble up also).
Pour the batter evenly into the two pans or muffin tins. Bake for about 30 to 40 minutes for cake pans and about 20 minutes for muffin tins. Test if they are cooked with a small knife or toothpick, it should come out clean when cakes are done.
Meanwhile, to make the sauce, combine the butter, brown sugar, heavy cream and vanilla extract in a medium saucepan. Bring to boil and then reduce to simmer gently for a minute or two until thickened and well blended.
When the cakes are done, poke little holes all over with toothpick, this will enable the sauce to be absorbed more easily. Pour the caramel sauce over cakes while both are still warm and leave to soak for about 10 minutes. Turn the cakes out upside-down onto serving plates (the bottom is the most sticky bit!).