Preheat the oven to 220°C/425°F/Gas mark 7. Lightly grease a 13 by 9 inch Swiss roll tin and then line with baking parchment.
Place the eggs and sugar in a large bowl. Whisk with an electric mixer on high until the mixture is light and frothy and the whisk leaves a ribbon train when lifted out of the bowl. Sift the flour into the mixture, carefully folding it in at the same time. Do not over mix or your cake will not be as light as it could be. Turn into the prepared tin and gently shake it to level it out, making sure that it spreads into the corners.
Bake in the preheated oven for about 10 minutes, or until the sponge is golden brown and has begun to shrink from the edges of the tin.
While the cake is cooking lay a piece of parchment paper on the counter top larger than the tin the cake is cooking in. Sprinkle evenly with several tablespoons of caster sugar.
Invert the baked cake onto the sugar covered parchment paper. Quickly loosen the paper on the bottom of the cake and carefully peel it off. Trim one short edge of the sponge with a sharp knife, about 1 inch in from the edge, making sure you do not cut it all the way through. Leave to cool for several minutes.
For the buttercream beat the butter or margarine and icing sugar together until well blended and smooth.
Spread the jam and buttercream evenly over the cake. If the cake is too hot it will absorb too much of the jam. Roll the cake up firmly, starting at the scored edge. Allow to cool before slicing with a serrated knife.
Recipe courtesy of Marie Rayner, The English Kitchen