Preheat the oven to 200'C.
Bake the potatoes in the oven until cooked (this should take about 45 minutes). Cut in half and scoop the flesh into a bowl. Allow to cool to room temperature before adding the flour and egg yolks. Season with salt and pepper and then mix gently until you have a dough. Roll into long thin sausage shapes and cut into 1cm pieces. Roll over a gnocchi roller or just use the back of a fork for this. Make sure you are rolling over a lightly floured tray so the gnocchi does not stick.
Bring a sauce pan of water to a simmer and cook the gnocchi. When the gnocchi rise to the surface they are cooked.
Remove from the pan and straight into ice water. Drain the gnocchi well before adding to the sauce.
Melt the butter in a pan then stir in the flour. Cook for 1min then gradually whisk in the milk. Slowly bring to the boil, whisking all the time, then simmer for 3-4 mins until the sauce is smooth and thickened.
Stir in the fontina, taleggio and gorgonzola cheeses and simmer the sauce, stirring all the time, until the cheeses have melted. Season lightly with salt and freshly ground black pepper.
While the sauce is simmering, boil the gnocchi in a large pan of lightly salted water for 3-4 mins until they just rise to the surface, or according to the packet instructions. Drain well and place in a shallow gratin dish.
Pour over the sauce and scatter over the grated Parmesan cheese. Place under a medium-hot grill for 4-5 mins until golden and bubbling.
«Add some smoked bacon to the sauce to enrich the flavour. »