1 cup superfine sugar (also called fruit or quick-dissolve sugar)
For the cake:
Preheat the oven to 325 °F (160 °C).
Sift the flour and granulated sugar twice and set aside.
Whip the egg whites with the cream of tartar and salt until foamy, then gradually add the icing sugar (lower the speed when adding), whipping until the whites hold a medium peak when the beaters are lifted. Stir in the vanilla.
Add the flour to the whipped whites in two additions and using a whisk to fold in the flour evenly and easily. Scrape the batter into a 25 centimeter ungreased tube pan, spread it to level and bake the cake for 30-35 minutes, until it springs back when gently pressed (try not to open the oven before 25 minutes).
Turn the cake pan upside down to cool the cake, and do not remove the cake from the pan until completely cooled. To remove, run a palette knife carefully around the outside of the cake, and very gently around the inner tube, invert the pan and tap it on the counter until it comes out.
Serve the cake with whipped cream and berries, if you wish. The cake will keep, well wrapped (not refrigerated) for up to four days.
For the mascarpone cream:
Whip 1/2 cup of the whipping cream until it holds a soft peak.
Stir the remaining 1/4 cup of cream into the mascarpone cheese by hand, along with the lemon zest and sugar until the sugar has dissolved.
Fold in the whipped cream and the lemon juice and chill until ready to assemble.
For the fruits and flowers:
Spread the berries out on a paper-towel lined tray and let them come up to room temperature. Be sure the flowers are completely dry, and for larger flowers, have an empty egg carton ready.
Use a small paintbrush to paint a sheer layer of egg white onto the berries and flowers. Some berries, like the raspberries and blackberries, do not need the egg white but can be dipped in the sugar directly.
Immediately after brushing, sprinkle or dip the fruits and flowers in the superfine sugar and shake off any excess.
Set the fruits and small flowers on the tray to dry for an hour, and large flowers can be set in the egg carton (so the air circulates all the way around them). Once dried, you can begin to decorate your cake.