Preheat the oven to 160ºC.
Spray a non-stick 4-hole Yorkshire pudding tray (each hole 10cm in diameter), with low-calorie cooking spray. Use to grease the holes thoroughly, then line the base of each with a disc of non-stick baking paper.
Place the eggs, granulated sweetener and caster sugar in a large mixing bowl. Whisk with a handheld electric whisk for about 10 minutes, until pale, thick and creamy. The mixture should be thick enough so that 'ribbon' trails are left on the surface when the whisk is removed.
Sift the flour into the mixture. Using a large metal spoon, gently fold in until the flour is just mixed in and the mixture is lump free (don't over mix the mixture, otherwise you will knock out the air you have incorporated!).
Divide half of the plain mixture between the 4 holes in the lined Yorkshire pudding tray and gently spread out.
Divide the chocolate mixture in half and place in a swirl on top of the plain mixture in the Yorkshire pudding tray.
Place in the preheated oven for 10 minutes, until golden and risen. The sponge should spring back when lightly pressed and no indent should be left.
Remove from the oven and leave to cool in the Yorkshire pudding tray for about 5-10 minutes. When cooled a little, use a round-bladed knife to run around the edge and gently ease the sponges out of the tray. Place on a cooling rack to cool completely, then carefully peel off the baking paper from the bottom of the sponges.
Wash the Yorkshire pudding tray, then grease and line again. Use the remaining plain and chocolate sponge mixture to fill the 4 holes again as before.
Bake, cool completely and remove the lining paper as before.
Take the tub of ice cream out of the freezer and remove the lid and foil seal. Place on its side on a chopping board and snip through the cardboard rim with a pair of scissors. Pull open the seam on the tub to expose the ice cream then discard the cardboard tub.
Working quickly, use a large, sharp knife to slice the ice cream into 4 thick slices about 2cm wide.
Quickly sandwich each slice of ice cream between 2 sponge rounds.
Place the finished ice cream sandwiches in a single layer on a baking tray or large plate and place in the freezer for about 15 minutes, just to firm up a little before serving. Serve and eat at once.