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Start by making the mousse layer. Grease a 16cm hemisphere tin with sunflower oil. Soak the gelatine in cold water to soften it. Meanwhile, whip the cream until billowy and just holding its shape, but not stiff. Put the lemon curd in a pan with the lemon juice, stir until smooth then bring to the boil. Take off the heat and leave to cool for 1 min then squeeze the excess water from the gelatine and add to the curd mixture, stir until the gelatine has dissolved then pour into the whipped cream. Beat until the mixture is smooth then pour it into your prepared hemisphere mould. Chill in the fridge for 1 hr or until set.
While the mousse is setting make the sponge base. Heat oven to 200C/180C fan/gas 6. Line a swiss roll tin with baking parchment and grease well with butter. Whisk the eggs and sugar together with an electric hand whisk for 2-3 mins or until thickened and pale.
Fold the flour, baking powder and lemon zest into the egg mixture with a large metal spoon until there are no pockets of flour visible. Gently tip the mixture into your prepared Swiss roll tin and bake in the oven for 15 mins or until lightly golden and springy to the touch. Allow to cool then cut a circle out of the sponge mixture which is big enough to fit snugly into your hemisphere tin on top of the mousse.
When the sponge is completely cold, spread it with the lemon curd and sit in on top of the set mousse (curd side down) and press down very gently so it sticks. Carefully turn the mousse cake out of the mould so that the mousse is now at the top, and sit it on a wire rack on top of a baking tray. Put the mousse cake in the freezer while you make the mirror glaze.
To make the mirror glaze soak the gelatine leaves in cold water while you heat up the curd and lemon juice in a small pan. Add the strip of lemon zest and bring to the boil. Take it off the heat and allow to cool for 1 min then remove the lemon zest, and add the gelatine as before, squeezing out the excess water first. Pour the glaze through a sieve into a jug and stir occasionally as it cools. When it has cooled down and is starting to thicken, but can still be poured, take the mousse cake out of the freezer and pour all of the glaze slowly over the surface. The excess will run off into the tray below the wire rack - this is meant to happen. You can then pour it back into the jug and glaze again if you like.
Softly whip the cream for the decoration, and transfer to a piping bag then pipe dots or spirals tightly a long the edge where the sponge meets the mousse cake. Cut the lemon slices in half and place them in the middle to decorate. Keep in the fridge until you want to serve it. Can be made the day before.
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