For the sponge:
Preheat the oven to 350°F. Grease a rectangular loaf pan. Mix the flour, cornstarch, baking powder, and salt in a bowl, and set aside. In a different bowl, beat the butter until soft and creamy, then add the cream cheese and beat until well combined.
Add the cream and keep beating, then add the sugar and eggs one at a time. Add the dry ingredients and mix until well combined. Put the mixture in the greased baking pan. Bake for 40-45 minutes at 350°F or until you can poke it with a toothpick and it comes out clean. Allow to cool.
Put the mascarpone cheese and sugar in a bowl and beat them together until soft and creamy. Then add the heavy cream and keep beating with an electric beater until thick and creamy. Add the lemon juice, and the mascarpone cream is done.
Add a thick layer of mascarpone cream on top of the sponge, and then put the blueberries, blackberries, and lavender sprigs on top.
You may use mint leaves for decoration.