Vegetarian recipe ideas
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Heat the oven to 180C/fan 160C/gas 4 and butter the sides and bottom of a 20cm square tin. Cut a 20cm × 40cm piece of baking parchment and the same of foil. Butter the foil and then put the baking parchment on top so that they’re stuck together.
With the foil-side facing up, fold in half widthways like a book, then open out, turn over and push up the centre fold to make a 4cm pleat, that sticks up, down the centre.
Line the base of the tin with the foil-side down, ensuring the pleat is uppermost and is dead in the centre of the tin, to create 2 identical rectangles within the tin. Ensure the paper-lined foil is pushed into all the corners of the tin with a slight overhang on either side.
Put the flour, sugar, butter, eggs and baking powder in a bowl and whisk until just combined. Divide the mixture between two bowls. Whisk the lemon zest into one plus enough colouring to make a strong custard colour. To the other, add the orange zest and enough colouring to make quite a bright orange colour.
Spoon each mixture into seperate halves of the tin. Make sure the divider is straight before smoothing the surface of each.
Bake in the oven for 25 minutes until the cakes have risen and are springy to the touch. Allow to cool for a few minutes before turning out onto a cooling rack and removing the divider.
Meanwhile, make the icing by beating together the butter, icing sugar, thyme, zest and juice until fluffy and smooth.
Trim the brown off long edges of the cakes and then cut each cake into 3 lengthways, ensuring all 6 strips are the same size and shape so that the cross section of each is a square.
Put a lemon, orange and then another lemon strip next to each other and use the icing to stick them together. Spread some more icing over the top and then stick the 3 remaining strips on top, alternating them in a chequerboard effect icing in-between them and on top.
Roll the marzipan out, on a clean worksurface dusted lightly with icing sugar, to a rectangle 20cm × 30cm or the same length as the cake and 4 times its width with a little extra to trim away.
Invert the cake onto the marzipan, iced-side down and 2cm in from the edge. Spread the remaining icing over the rest of the cake, apart from the chequerboard ends.
Carefully roll up the cake in the marzipan and seal where the marzipan joins. Trim any ends off to neaten.
Put the cake, seal-side down on a plate. Crimp the two top edges and score a diamond pattern on the top using the back of a knife before serving.