Preheat the oven to 160°C (320°F) with fan. Grease a 20x30 cm (8 x 12 inch) sheet pan and line it with baking paper.
In a large bowl, beat the eggs and sugar on high until the mixture becomes pale in colour and triples in volume.
Add the oil, double (heavy) cream and lemon zest into the eggs, mixing to incorporate. At this stage you can also add any food colouring. *
Sift in the flour, baking powder and salt. Gently fold the dry ingredients into the wet using a spatula. Be gentle, so you don't deflate the air we whipped into the eggs.
Pour the batter into the lined tray and level it using a spatula. Rap the tray on the work surface a few times to remove any bubbles. Bake for 12 minutes on until a skewer comes out clean.
Once baked, let the cake cool for 5 minutes, then place it face down on baking paper. Peel off the bottom layer of baking paper (used to line your tray).
When the cake is cooled, use a 2.5-inch(6.5 cm) cookie cutter to cut out 15 circles. These will be your cake layers.
White chocolate cream cheese frosting
Melt the white chocolate in the microwave. Start with 30 seconds, then give the chocolate a quick mix. Return to the microwave for a smaller interval of 15 seconds, mixing after each time. Repeat until the chocolate is fully melted, then let the chocolate cool down to room temperature before using it.
In a medium bowl, beat the butter and salt together using a hand mixer or stand mixer, until the butter becomes paler in colour.
Add the cooled melted chocolate and cream cheese over the butter mixture . Mix everything for 2-3 minutes until smooth and airy.
Divide the cream cheese frosting into two and add the blueberry jam to one half. Mix well to combine. If needed, add 1-2 drops of purple food colouring. Place the frosting in a piping bag fitted with a wide star (or round) nozzle.
Assembling the mini cake
Place one 2.5-inch (6.5 cm)circle of sponge on your plate. Pipe small dollops of cream cheese frosting on the inner perimeter or the circle, forming a ring. Add roughly 1 teaspoon of blueberry jam inside the circle, ensuring it doesn't go over the cream cheese ring.
Add a second layer of sponge ontop and repeat the process with the frosting and jam.
Place the top layer of sponge, then decorate the entire top with dollops of cream cheese frosting. Decorate with fresh berries, mint leaves and edible roses or flowers. Enjoy!
Notes
I recommend using a kitchen scale in grams for more accuracy. The cups used for the conversion are standard US customary cups (1 cup flour = 136g). There are many different types of cups across the globe, which is why I strongly recommend using grams instead.
I recommend you use either powder or gel food colouring for this recipe. Liquid colouring can be used as well, but the resulting colour will not be as vibrant.
If you don't fancy the lemon sponge, leave out the lemon zest and replace with flavouring of your choosing (vanilla, chocolate etc).
Having two colours cream cheese frosting is entirely optional. You can use either only plain frosting or add blueberry jam in the entire portion.
Cream cheese can also be replaced with mascarpone.
Blueberry jam can be substituted with other jams, preferably berries: strawberry, raspberry etc.