Grease a 15 x 10 x 1 in. (37 x 25 x 3 cm) jelly-roll pan; line it with wax paper; grease the paper.
Measure the flour, baking powder, and cinnamon into a sifter.
Beat the eggs in a medium bowl until foamy; gradually beat in the sugar and acesulfame-K until the mixture is very thick and light.
Stir in the vanilla.
Sift the dry ingredients over the egg mixture; gently fold in until no streaks of flour remain.
Spread the batter evenly in a prepared pan.
Bake in a 375°F (190°C) oven for 12 minutes or until the center springs back when lightly pressed with a fingertip.
Loosen the cake around the edges with a knife; invert the pan onto a clean tea towel.
Peel off the wax paper.
Starting at the short end, roll up the cake and towel together.
Place the roll, seam-side down, on wire rack.
Cool completely.
Combine the prunes and water or apple juice in a small saucepan; simmer, covered, for 25 minutes.
Puree the prunes with the remaining cooking liquid, a bit at a time, in a blender or food processor.
Stir in the lemon juice and cinnamon.
Chill.