Preheat the oven to 350 degrees. Butter and flour two 9 inch round cake pans.
Place the dates in a large saucepan with 3 cups cold water. Bring to boil, stirring a little to break up the dates. Then leave to simmer for 1 minutes before removing from heat. Stir in the baking sode causing 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 the 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 the bowl in between mixing. The dough will now be quite watery but don't worry! Finally add the baking powder.
Pour the batter evenly into the two pans. Bake for about 30 to 40 minutes. Test if they are cooked with a small toothpick which should come out clean when 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 blended well. Cool a bit to thick and pour over the cakes spread to the edge of the cake.