In a bowl combine flour, cornstarch, baking powder and salt. Mix to blend and set aside.

In a large mixing bowl beat eggs and sugar until the mixture gets pale and thick and increases its volume. Add vanilla extract and olive oil and beat shortly until absorbed.

Gradually add the dry ingredients, mixing gently with a rubber spatula, until incorporated.

Transfer the batter to the prepared pan and bake for 40-50 minutes or until a wooden skewer inserted in the centre comes out clean. Let cool until room temperature before assembling the cake.

In a large bowl beat together mascarpone, sour cream and powdered sugar until fluffy.

Melt both kinds of chocolate in a double boiler or in the microwave (very carefully at low power, mixing every few seconds). Mix the melted chocolate until the colour becomes uniform.

Using a rubber spatula carefully fold the chocolate into the mascarpone mixture.

Using a serrated knife cut horizontally the cake base in 3 or 4 equal layers. Place one layer on a serving plate and cover generously with frosting. Arrange on top part of the berries (½ or ⅓, depending on the number of layers). Repeat the procedure until finishing all layers.

Cover the cake on top and around the sides with the rest of the frosting. Decorate with berries.