Once the cake has completely cooled and you are ready to serve, you can assemble the cake with the frosting and berries. (The frosting can be made in advance and kept in the fridge until you are ready to use).
The easiest way to make the frosting is by using a stand mixer. If you do not have a stand mixer, you can use a hand held electric mixer. Or, you could do it by hand with a whisk, but this will take longer and require a bit of muscle!
Ensure that the mascarpone cream and whipping cream are at the same temperature. It will be easier to whisk if they are both at room temperature but you can still make the frosting when the ingredients have just come out of the fridge, it might just take a little longer to become smooth and fluffy.
Place the mascarpone cream into the bowl of the mixer and use the balloon whisk attachment to gently whisk, turning up to a medium power for a few minutes.
Gradually pour in the thick whipping cream with the motor still running and continue to whisk for a further few minutes until it all becomes light and fluffy and increases in volume.
Drizzle in the maple syrup and then stop the motor to taste for sweetness. Add a TBSP of powdered sugar, whisk and taste again. Continue doing this until you reach the level of sweet you prefer depending on how much sugar you want in the recipe. Stop the motor once the mixture is light and fluffy and fully combined at the level of sweet you prefer.
Add half of the frosting to a pipping bag with a wide open nozzle and squeeze on top of the bottom layer of the cake until it is evenly covered.If you do not have a pipping bag, simply use a small pallet knife to smooth the mixture on top.Add a few blueberries around the edge of the cake (optional). Place the 2nd layer of the cake on top of the frosting and repeat with the frosting for the finishing layer.
Pile the cake high with blueberries! If you find some of the blueberries start to topple, you can fix a few of them in place using tooth picks - but you need to warn your guests of them or simply remove for serving.
Store any leftover cake in the fridge in an airtight container with a piece of kitchen roll in the bottom. It needs to be stored in the fridge as the frosting must be kept chilled for it to remain fresh. Eat within 3 days.