Line a 9 inch cake pan with baking parchment.
Preheat the oven to 180C Fan.
Put the butter and 140g chopped dark chocolate into a medium heat-proof bowl. Set over the pan of barely simmering water, ensuring the bottom of the bowl doesn’t come into contact with the water (or the chocolate will overheat) and allow it to slowly melt, stirring often until smooth (alternatively, pop into a microwave-safe bowl and melt in 20 second bursts, stirring between each - this is the quickest method but you have to watch closely and ensure it doesn’t overheat). Set aside to cool slightly.
Meanwhile, place the eggs and sugars in a large mixing bowl and beat with an electric whisk on medium-high speed for about 1 minute - or until pale, thick and roughly doubled in size.
Add the chocolate-butter mixture and vanilla extract and mix until combined.
Sift in the flour and salt, then fold in until you have a smooth batter. Pour into the lined cake tin and bake for 15 minutes - or until the top is puffed up. It will continue to set as it cools. Remove from the oven and allow to cool in the tin.
Whisk the double cream until you have stiff peaks. This is easiest with an electric whisk. It should take about 3 minutes. Nothing much will happen to begin with and then it will thicken very quickly so take care not to over whisk it or it will turn grainy.
In a separate bowl beat the mascarpone with a wooden spoon until it's lovely and smooth and completely lump free. Place a sieve over the bowl and sieve in the icing sugar. Stir to combine and add the vanilla
Add the whipped cream and gently fold it all together. Be gentle as you don't want to lose all the air in the whipped cream.
Pour half the filling on top of the cooked brownies.
Dip the sponge fingers into the coffee mixture and arrange in a single layer. Top with the remaining cheesecake mixture and and smooth it out before popping in the fridge overnight. This is important as it needs enough time to set of you will have a melty mess and no one wants that.
Add the double cream and ovaltine to a bowl and whisk to stiff peaks taking care not to overwhisk. Use a piping bag to pipe peaks of cream on top of the cheesecake. Pop back in the fridge to firm up or you can serve straight away
Before serving dust the top with sieved cocoa powder. And enjoy!!