Measure out 25 ml orange juice. Use the remaining juice for other purposes. Boil 25 ml orange juice and 100 ml milk in a pot. Mix remaining milk with pudding powder and 1 tbsp. sugar. Stir the mixture into the hot milk and let it simmer for about 2 minutes while stirring.
Pour into a flat container and cover the pudding directly on the surface with cling film. Leave to cool until the pudding is at room temperature.
Mix flour, baking powder and orange peel. Beat the eggs, salt and 250 g sugar with the whisk of the hand mixer until thick and creamy. Briefly stir in the flour mixture. Let the oil slowly run in while stirring.
Stir in mineral water briefly. Spread the dough on one half of a deep, greased and flour-strewn baking tray (à 36 x 41 cm; 3 cm high). Use a strip of aluminium foil folded several times to separate the dough.
Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for about 25 minutes.
Cut the number template to size. Take the cake out of the oven and turn it over onto a board covered with baking paper. Let it cool down. Place the number on the cake and cut it to size. Use cake remains for other purposes.
Whisk butter and icing sugar with the whisk of the hand mixer until creamy white. Stir in the pudding bit by bit. Halve the number of cakes in the middle and spread the lower half of the cake with approx. 1/3 butter cream.
Put the upper half back on and press down a little. Spread a thin layer of the remaining buttercream all around and chill for about 30 minutes.
Dye 400 g fondant with blue food colouring light blue and roll out thinly on a work surface dusted with icing sugar (approx. 35 x 40 cm). Cover the cake with it, cutting off the protruding wheels.
Using a densely serrated pastry card, press a chequered pattern into the fondant, lightly carve the overlaps crosswise with a small knife. Brush the cake thinly with a little water and decorate the crosses with sugar pearls.
Roll an even strand of dough about 1.20 m long from the remaining white fondant. Fold over the edges of the cake.