Lightly butter 2 20cm sponge tins and line the bases with baking paper.
Separate eggs. Beat egg whites with boiling water until stiff. Add Chelsea White Sugar slowly. Beat until dissolved. Mix in yolks one at a time. Sift flour, cornflour and baking powder into the sugar and egg mixture. Fold in.
Divide mixture between sponge tins and bake for 20–25 minutes. Drop sponges (in tins) on bench when taken out of oven.
Leave in tins for 10 minutes, then turn out onto a wire rack to cool completely.
In a small saucepan, bring Chelsea Caster Sugar, water, lemon juice and sherry to a boil, for 1–2 minutes, stirring to dissolve sugar. Turn off heat and let syrup cool. In a bowl, combine jam with 3 cups berries, mashing slightly.
In a large bowl, whip cream and Chelsea Icing Sugar to stiff peaks.
Brush cake with syrup and slice into roughly 2cm thick pieces. Place half of the sponge slices snugly into a trifle dish or glass bowl. Spread half the berry mixture then half the whipped cream over this first layer. Repeat to make one more layer. Garnish with remaining cup of berries and refrigerate until ready to serve.