Remove eight outer leaves from the fennel bulbs and set to one side. Finely chop the rest of the bulbs, with the exception of the bitter core. Bring the vegetable stock to the boil, add the whole leaves, cover and remove from the heat. Leave for ten minutes and then remove the leaves from the stock. Heat the olive oil in a pan, add the rice and stir until every grain is coated with hot oil. Then add the chopped fennel, pour over the wine and cook over low heat for three minutes, stirring continuously. Continue cooking, gradually adding the reserved vegetable stock, until you obtain a creamy consistency. Remove from the heat, stir in the butter and serve on the fennel leaves.