You'll need a large flat tray. Preferably about 40 cm x cm. Set the oven to 180 C
Chop the dates and put them into a small pot with the water. Bring up to a boil and add the bi carb . When it fizzes turn off the heat. Break up the dates with a fork until they are a smoothish puree.
Set aside to cool
Beat the softened butter with brown sugar together till light and creamy. Add an egg and mix well scraping down the bowl. Add 2 tablespoons of flour and stir
Add the second egg and mix well. Add the cooled date puree and the rest of the flour. The mixture will be smooth and creamy.
Spread the mixture onto the sprayed and paper lined tray right to the edges evenly right across the tray.
Bake the sponge for 10 to 15 minutes or until it springs back when touched with the tips of the fingers. Set aside fior several minutes.
Sprinkle the sponge generously with caster sugar and put a piece of baking paper ( or a clean teatowel) on top. Top this with another tray and flip over so that the sugared side is now at the bottom. Roll up the warm sponge using the paper (or tea towel) and leave rolled for about 15 minutes or until it is just barely warm. The sponge may possibly crack when rolling. Don't worry too much this does happen. Be gentle but firm.
Meanwhile, whip the cream with the brown sugar and vanilla till stiff peak. (This means till it is quite firm and spoonable)
Unroll the cake and spread thinly with cream and roll up again. Push any cracked bits together and secure the ends tightly. Put it onto a tray and refrigerate for several hours or overnight