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Place the egg whites and sugar in a spotlessly clean heatproof bowl (the bowl of your stand mixer if you have one) and place the bowl over a pan of gently simmering water, make sure that the base of the bowl doesn't touch the water.
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Gently whisk the eggs (with a hand whisk, not an electric one) until the sugar has dissolved, if you rub the mixture between your fingers it should feel smooth with no hint of graininess. It should measure 60C/140F on a thermometer.
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Remove the bowl from the heat and whisk with an electric mixer/stand mixer on high until the egg whites have formed a stiff, glossy meringue and the mixture is completely cold, the base of the bowl should feel cool to the touch, this will take about 10 minutes.
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Turn the mixer down to low and gradually add the butter, a tablespoon at a time, mixing to incorporate after each addition. The mixture may curdle or begin to look soupy, just keep on whisking and it will come back together.
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Once all of the butter has been incorporated and the buttercream is smooth and silky, measure out 120ml (1/2cup) of the cooled roasted strawberry puree. Mix it into the buttercream gradually, a spoonful at a time. The mixture shouldn't split, but if it does don't worry! Just place a couple of heaped tablespoons of the buttercream in a small bowl and microwave it until it has melted, then drizzle it back into the rest of the buttercream slowly while whisking on a high speed, after a couple of minutes of whisking it will come back together.
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Assemble
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Turn the cooled cakes out of their tins and place one layer on a cake stand or serving platter. Spread just over a quarter of the buttercream over the cake and top with another layer, pressing down on it gently; spread over just over another quarter of the buttercream and top with the final cake layer. Place the cake upside down so that you get a flat top; spread most of the remaining remaining buttercream on the top of the cake, spreading it flat, then use the rest of the buttercream to fill in any gaps around the edges and to coat the whole cake in a crumb coat.
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Place the cake in the fridge for about 30 minutes then use a palette knife dipped in hot water to smooth out the icing.
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Return the cake to the fridge while you make the ganache.
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Scrape the remaining roasted strawberry puree through a sieve to remove the pips and measure out 65g worth, place it in a small saucepan and bring it up to a simmer. Remove from the heat and add the chopped chocolate, stir until smooth then add the elderflower cordial. Mix in a little pink gel food colouring if desired. Set aside until it has cooled but is still pourable.
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Pour the ganache into the centre of the cake and use a palette knife to tease it towards the edges so that it drips over; return the cake to the fridge for 30 minutes until the ganache has firmed a little (it doesn't set hard like regular ganache).
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Let the cake come up to room temperature and decorate the top of the cake with extra strawberries just before serving. If you use sliced strawberries to decorate, the moisture will seep out into the ganache and make it go a little melty - I suggest using whole ones, or if you want to decorate it as I did with strawberries standing on their ends, then I recommend dipping the sliced ends into melted chocolate and allowing it to set before decorating the cake, this will provide a barrier to stop the moisture from seeping out.