Can you top it?

Chocolate, spaghetti, plums, bananas, mashed potato......

When is a pizza not a pizza? When it's made of brioche and covered with pasta? When it's square and the dough is pumped full of melted cheese? Take a look at what now passes for pizza on the world's menus and you'll find everything and anything goes. It's enough to make the originators of the Neapolitan treat spin in their graves.

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Coca chocolate pizza

Spanish savoury sweet pizza

When is a pizza not a pizza? When it's made of brioche and covered with pasta? When it's square and the dough is pumped full of melted cheese? Take a look at what now passes for pizza on the world's menus and you'll find everything and anything goes. It's enough to make the originators of the Neapolitan treat spin in their graves.

Surfing the Internet you will find numerous sites devoted to the subject of weird pizzas – the most jarring toppings, such as mashed potato with mayonnaise (the Monterey), spaghetti, seaweed and squid ink all hailing from Japan. In fact, what is most conspicuous is the absence of tomato and cheese from the Japanese pizza scene; the tomato is barely known in Japan, while cheese is a luxury novelty, appealing to those wishing to imitate European food habits.

But chocolate pizza, that's here and now in London at Alain Ducasse's Michelin-starred restaurant, Spoon. While Banana- and plum-topped pizzas are on the menu at Tartuffe in Islington and Clapham. And a PizzaExpress subsidiary serves everything from Thai chicken to Chinese crispy duck.

For the big chains, however, innovation is not what goes on top but the very form of the pizza itself. Pizza Hut – its £300m annual turnover makes it the market leader in the UK's £1.9bn pizza market – thought that it was being daring this May when it launched the Quad, a home-delivered pizza that gives you four topping choices on the one base (none of them with mashed potato, plums or squid ink, though).

Pizza Hut is a massive worldwide pizza chain, with 30,000 outlets in 30 countries, and can justifiably claim to be the innovators in the British market. In a bid to keep one step ahead it has also introduced the Stuffed Crust (the base stuffed with cheese), the Edge (the first crustless pizza) and, last year, the Twisted Crust, a pizza with a detachable garlic and herb flavoured crust which you can pull off and dip into a spicy sauce.

But it is its main rival, PizzaExpress, which has made the first tentative step towards alternative toppings. This year it bought the Gourmet Pizza Company, which has been pushing back the boundaries over the past five years. Nabil Mankarious, an Egyptian who heads this small group of five pizza outlets, sees the pizza as no more than a bread base. He explains that in the Middle East they have similar baked flat breads designed to carry whatever filling you most like – in Egypt spiced minced lamb, for example.

Nor are the French respecters of the Italian tradition. Bananas, plums, apples and berries are among the toppings offered at the London outlets of Tartuffe. The idea there is that you eat pizza the whole meal through, explains Phillipe Vonthron, one of the partners. Sharing with friends, you start with savoury and finish with sweet. "The word for pizza in French is tarte flambée," he says. "But the name has nothing to do with flaming in brandy. We use a light, thin dough. For the desserts we spread the pizza with French yoghurt and you choose your fruit topping."

Whatever we may think, the sweet pizza is not alien to Italians, says food historian, Anna del Conte. They go back to the 16th century. In fact, the classic Neapolitan pizza with cheese and tomato is comparatively new in terms of Italian pizza history.

These mozzarella and tomato pies were first sold on street stalls in Naples in the 18th century, says del Conte, and only achieved distinction after Queen Margherita of Italy, visited the city in 1889 and asked to try the dish everyone was talking about. She gave her name to the Margherita, which bears the colours of the Italian flag: red (tomato), white (mozzarella) and green (basil).

Sweet pizza abounds in Italy, says del Conte. Pizza rustica from Abruzzi is a pizza made with sweet pastry filled with a mixture of ricotta, prosciutto (ham) and mortadella with eggs and cinnamon. A northern Italian sweet pizza (pizza dolce), made for village fêtes, is several layers of dough enriched with ricotta, cinnamon and lemon zest and stuffed with walnuts, almonds, pine nuts, candied fruits and peel, grated chocolate, sugar and cocoa.

The first chocolate pizza in the UK may have been created by chef Stephen Terry at the Coast restaurant in London in the late 1990s. "I experimented with a part-baked puff-pastry base," says Terry, "and placed a metal ring over the middle and filled it with a chocolate fondant. After baking, I decorated it with shavings of white chocolate and candied pistachio nuts and a blob of white chocolate ice-cream." It was a great hit. Alas, Coast is no more but Terry has moved to the Walnut Tree Inn in Wales and is planning to resurrect his chocolate pizza there.

But great minds think alike. Now the French star, Ducasse, who has collected more Michelin rosettes than anyone else, has created his own chocolate pizza (see recipe on previous page). It's on the menu at Spoon in the Sanderson hotel and in Ducasse's new book Spoon Food & Wine (Conran Octopus, £25).

His base is a rich egg-and-butter based brioche, sweetened with cocoa and sugar, and sprinkled with cocoa before baking. Three-quarters of the way through it is dribbled with cream. When it turns syrupy he puts on the final topping of chopped chocolate to melt for two minutes. Delicious.