Preheat oven to 350°F (180°C). In small bowl, mix together with electric
mixer, flour and 3/4 cup (175 mL) each of the sugar and cocoa.
In large bowl, beat 1 3/4 cups (425 mL) of the egg whites until foamy. Add
lemon juice, vanilla, cream of tartar, and salt to egg whites. Beat until soft
peaks form. Still beating, gradually add remaining 3/4 cup (175 mL) of the
sugar until stiff, glossy peaks form.
Adding one-quarter at a time, gently fold flour mixture into egg whites just
until blended. Gently spoon batter into ungreased 10-in (25-cm) tube pan with
removable bottom. Run spatula through batter to eliminate air pockets.
Bake in centre of oven for 40 minutes or until cake springs back when
lightly touched. Invert pan; cool completely.
Cut cake in half horizontally to make two layers. Stir orange rind into frosting. Spread bottom layer evenly with frosting. Sprinkle with berries, if using. Replace top; frost cake completely. Garnish with mint and additional berries, if using.