Heat the oven to 200C (180C fan) 400F, gas 6.
Beat the egg yolks with 1/3 of the sugar, a pinch of salt and the lemon zest until pale and creamy. Beat the egg whites until stiff. Gradually add the remaining sugar and continue to beat until very stiff.
Mix the carrot, nuts and flour. Add 1/3 of the egg white to the egg yolk mixture and mix in well. Spoon the remaining egg white on top and sprinkle over the nut mixture. Fold in very carefully, using a wooden spoon.
Line the pan with baking parchment and pour in the dough. Smooth the top and bake for approx. 60 minutes. Allow to cool in the pan overnight.
Turn the cake out onto a sheet of baking parchment, so that the smooth underside is on top. Knead the marzipan paste with the confectioners' sugar until smooth. Color about 1/6 green, the rest orange (using the red food coloring). Divide into 16 pieces and shape into little carrots with leaves.
Dust the cake with icing sugar and decorate with the carrots.