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Slice 300g of the strawberries. Put in a pan with the icing sugar and lemon juice. Gently heat until the juices start to run, then cook, uncovered, for 2 mins. Tip into a food processor or mini blender and whizz to a purée, then press through a sieve to remove the seeds.
Grease and line the base and sides of a 23 x 33cm Swiss roll tin. Grease and line the base of a deep 20-21cm round cake tin. (You will also need a deep 23cm round cake tin for later.) To make the polka dots, whisk the egg and sugar using a hand-held electric whisk until thick, pale and the mixture leaves a trail when the whisk blade is lifted. Sift in the flour and fold in using a spatula until well mixed, then fold in 1 tbsp of the strawberry purée and a few drops each of the pink and red food colouring.
Spoon small dots of the mixture randomly over the base of the Swiss roll tin, leaving a little space between each dot. Place in the freezer for 20 mins until the pink dots are firm to the touch.
To make the cakes, heat oven to 180C/160C fan/gas 4. If you have a tabletop mixer, you can make the whole quantity in 1 batch. If using a handheld electric whisk, it will be easier to make the cakes in 2 batches of 4 eggs each. Whisk the eggs and sugar until the mixture is light and pale, and the whisk blades leave a trail when they are lifted – this will take 5-8 mins. Sift in the flour and baking powder, drizzle in the butter and vanilla, and fold everything together using a spatula until all the flour is incorporated.
Pour half the mixture into the prepared cake tin and the other half over the Swiss roll tin, spreading it out evenly. Bake the rectangular cake for 15 mins and the round cake for 25 mins. Turn out each cake onto a tea towel, then remove the paper and invert onto a wire rack to cool.
To make the filling, beat together the mascarpone, icing sugar and remaining strawberry purée. Chop 200g of the remaining strawberries into small pieces. Whip the cream to soft peaks and fold into the mascarpone with the chopped strawberries.
Line the base and sides of the 23cm round cake tin with cling film. Trim the edges of the rectangular cake, then cut into 2 x 9cm-wide strips lengthwise. Fit around the sides of the tin, trimming to fit if necessary. The cake should butt together fairly tightly.
Cut the round cake into 3 layers (you could cut it into 2 layers if you find it easier, and divide the following filling accordingly). Place the bottom layer in the base of the cling film-lined tin. Reserve 3 rounded tbsp of the filling in a small bowl, then spoon half the remainder over the cake, smoothing it flat. Place a second cake layer on top and add the remaining filling, smoothing it flat. Top with the last cake layer. Cover the cake with cling film and chill for 24-48 hrs, along with the small bowl of reserved filling.
Carefully remove the cake from the tin and peel off the cling film. Slide the cake onto a serving plate. Slice the remaining hulled strawberries. Spread the reserved filling over the top of the cake and fill with sliced berries, adding in the 8 whole ones.
Warm the apricot jam with 2 tbsp water in a small pan, then press through a sieve. Brush liberally over all the strawberries. Dust the top edge of the cake with icing sugar. Chill until ready to serve.