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Heat oven to 180C/160C fan/gas 4. Grease and line the bases of 2 x 22-23cm straight- sided, loose-bottomed square tins.
To make the sponges, put the egg yolks and sugar in a large mixing bowl and beat with an electric whisk until pale. Add the lemon zest and juice, vanilla, semolina, almonds and poppy seeds, then whisk again briefly to mix.
Clean your beaters, and beat the egg whites in a separate bowl to stiff peaks. Add a quarter of the egg whites to the other mixture and stir in well to loosen. Add the remaining egg whites and, using a big metal spoon, gently fold in until no big lumps of egg white remain. Divide the mixture between the tins, spread the top to smooth and bake for 20 mins until springy to touch and a skewer inserted into the centre comes out clean. Leave the sponges to cool in the tins.
When the sponges are cool, start to prepare the filling. Put 400g of the strawberries and the icing sugar in a food processor or blender and whizz to a really smooth purée. Transfer to a saucepan and gently warm. Meanwhile, soak the gelatine leaves in a bowl of cold water.
When the purée is hot but you can still put your fingers in it, squeeze out the excess water from the gelatine, take the purée off the heat and stir in the gelatine to melt. Pour into a bowl and put in the fridge to quickly cool. Whip the cream until it is thick and holding peaks.
Halve the remaining strawberries and trim so they’re the same length – so when they are sitting on the sponges, they don’t come above the top of the tin. Line up the strawberries all the way around the edge of one of the cakes, cut-sides flat against the side of the tin (see step-by-step). When the purée is cool, fold through the whipped cream until evenly mixed, then pile into the tin, spreading to fill all the strawberry corners, and smooth the surface.
Remove the remaining sponge from its tin and, using a ruler or skewers and a small heart-shaped cutter, cut 8 hearts from alternating ‘squares’ – as if your sponge is divided into a 4 x 4 grid (see step-by-step). Turn the cake onto the mousse to top, and peel away the paper. Gently press into the mousse to stick and chill for 5 hrs or overnight to set.
Just before serving, sieve the jam to remove any lumps, then use to cover the mousse in each heart hole – a small piping bag is good for this (see step-by-step). Finish by cutting 8 squares of baking parchment the same size as your grid squares were. Sit over the 8 heart ‘squares’ and dust heavily with icing sugar. Carefully lift off the parchment squares, leaving a chessboard pattern on the top, and serve immediately with cake forks and dainty cups of tea. Will keep in the fridge for up to 2 days.