Whip the egg whites and cream of tartar together until stiff peaks form.
Rub the sugar and lemon zest together, then add to the egg whites and whip together.
Gently fold in the egg yolks until well combined.
In a separate bowl, mix the milk, oil, vanilla essence, and lemon juice.
Sift the flour, baking powder, and salt together.
Gently fold the dry ingredients into the egg mixture.
Fold in the milk mixture until just combined.
Pour the batter into a prepared cake tin and bake until a skewer comes out clean.
Allow the cake to cool completely.
Make the filling: cook the blueberries, strawberries, lemon juice, sugar, and water over medium heat until soft and jammy. Let cool.
Whip the cream and icing sugar together until soft peaks form.
Break the cooled sponge into pieces and layer into cups with the blueberry filling and cream frosting, repeating until the cups are full.
Chill briefly before serving.