Sift the first five ingredients together twice; set aside.
Combine the egg substitute and vanilla extract, raspberry liqueur, and buttermilk.
Using an electric mixer, cream the margarine or butter and the prune puree.
Add acesulfame-K and sugar and beat well.
Gradually add the egg substitute alternately with the flour mixture.
Beat until well combined.
Beat the egg whites until stiff.
Add the cream of tartar and continue beating.
Add a small amount of egg whites to the batter to lighten it.
With a rubber spatula, fold in the remaining beaten whites.
Pour into two 8 in. (20 cm) round cake pans that have been coated with non-stick cooking spray.
Bake in a preheated 350°F (180°C) oven for 30-35 minutes.
Cool, then invert onto a plate.
Combine the raspberries and aspartame in a food processor to make raspberry puree.
Spread raspberry puree over the top of one layer.
Put the other layer on top and cover with raspberry puree.
Decorate top with whole raspberries if desired.