We all know about fairy cakes - but witches cakes? What do we know about those? I wouldn't have been addressing this problem if I hadn't volunteered to make a 20th birthday cake for Winnie the Witch.

On offering, I had in mind a small, circular chocolate cake with Winnie's broomstick in the middle surrounded by 20 red candles and perhaps a little model of her black cat Wilbur. But nothing is ever that simple.

Winnie the Witch is the creation of Oxford's award winning children's book illustrator Korky Paul, who was commissioned to illustrate Winnie the Witch by Ron Heapy, editor of Oxford University Press in 1987. It proved an immediate success. Everyone loved her.

They loved Wilbur, too, as he is a very special cat. Winnie can only see him when his eyes are open because his eyes are green and everything else in her house is black. So when Wilbur closes his eyes, Winnie always falls over him.

Why I assumed that I could get away with presenting a simple little cake to Winnie, I really don't know. I should have realized that Korky Paul's imagination stretches far beyond mundane chocolate cakes decorated with broom sticks, candles and cats. When Korky Paul thinks cakes - he thinks big magical cakes, glistening with rainbow-coloured icing.

The subsequent illustration of Winnie's cake by Korky, therefore, which appeared in my email system shortly after my offer, should have come as no surprise.

But it did. I still remember the moment I pulled the illustration out of the email and enlarged it on my screen. I believe the word 'gobsmacked' comes into the equation somewhere along the line.

How on earth could I reproduce that? It was monstrous. It had eight tiers. It was at least three feet high. But I'd promised. Besides, this week is the 100th anniversary of Oxford University Press's Children's Book publications. Winnie the Witch features large on OUP's wide range of books published to engage a child's mind and capture its sense of the absurd. They were the first publishers to publish books for young readers with no moral lesson, that could be enjoyed for their own sake. That birthday had to be celebrated, too, and what better way to highlight this occasion than through a birthday cake for one of OUP's most popular characters.

Fine - but how on earth was I to make it? I haven't an oven large enough for the bottom layer, made from chocolate, and the layer of fruit cake, which is heavier than chocolate sponge, would press into the bottom layer and squash it.

By the time I had added the layer of rainbow cake, the cheesecake and the strawberry shortcake and ginger sponge cake, the orange cake and the Black Forest cake, the whole delectable edifice would come tumbling to the ground.

It was a friend who gave me the necessary confidence. He reminded me that witches had magical powers - that they could wave their wand at anything and turn it into something else. So, he had suggested, why not make a model of the said cake using cardboard, wood, plaster and royal icing sugar and leave it to Winnie to do the rest.

Every time she wanted to offer the children and her cat Wilbur a slice of cake, she could magic it into a sticky edible substance and then magic it back into a sculpture once again.

She could even magic back the slice that had been cut - she's that kind of witch.

So that's what I did. Thanks to our maintenance manager Nigel Waters (winner of Gannett's 2007 Unsung Hero Award) I was in business. It was Nigel who kindly made me the bases for each the cakes using wood, very, very firm cardboard and a few nails.

Having piled these cardboard cake bases into my faithful Mini, I headed for my cottage in Eynsham, stopping off at Evenlode DIY on the way. If anyone knew what kind of plaster I would need to make this thing work, it would be Robin Saunders. His Aladdin's cave of a shop stocks everything from single nails to strips of wood cut to order, and all types of glue and plaster. We talked plaster for about half an hour and I left armed with a small sample bag, costing just £1.60, which provided enough special adhesive plaster to coat the entire eight tiers. I don't know where else I could have got such service.

I worked layer by layer, allowing each a day for the plaster to dry before coating it with the relevant coloured icing. The ornaments were created from salt dough, which I dyed first with cake colourings, then baked for 24 hours in a slow oven.

Gosh, what fun I had once the cake began to look like a cake. People would pop round to the cottage just to watch its progress, and progress it did.

I admit being worried that my ginger cat would leap up on to the kitchen table and give it a rub or two. But no - he kept well out of the way, probably scared I would cover him with plaster, too. Or perhaps Wilbur was magicking him out of the way.

I won't describe the second fear that overcame me. Enough to say that creating an eight-tiered model of a witches cake that is more than three foot high proved easy enough, but how on earth was I going to get it to the office? Should I tie it on to the top of the Mini and hope for the best, or cry for help?

Nigel sorted that one for me, too. Thanks to him, it travelled in state at the back of one of our delivery vans, perched firmly on a large pile of The Oxford Times and a few Witney Gazettes.

So what goes into a witches cake? Cardboard, wood, icing sugar firmed with egg whites, salt dough and just a little bit of culinary magic. Happy Birthday Winnie.

You can see Winnie's cake at the Oxford Literary Festival and meet Korky Paul, too. He will be holding a children's workshop on Sunday, March 25, in the Hall at Christ Church at 4.30pm.