Preheat oven to 400 F. Warm Butter Olive oil, whisk flour into oil and increase heat to medium. Cook, stirring for 1-2 minutes. Whisk in milk and bring to a boil, stirring. The sauce will thicken slightly. Whisk 2 eggs into the mixture and cook, stirring for 1-2 minutes more until sauce has thickened. Add grated cheese and Parmesan and stir until melted. Remove from heat and season with salt and pepper to taste. Roll out 1 sheet of puff pastry on a floured surface to 12 inches square. With a sharp knife, cut dough into 12 - 3 x 4 inch rectangles. Mix egg yolk with water and brush edges of half of the rectangle with the egg mixture. Place a piece of ham in the center of the rectangle and top with 2 tablespoons of the cheese mixture. Cover the ham mixture with one of the plain rectangles and firmly press down the edges to seal. Repeat with the other rectangles and filling. Prick several times with a fork and brush tops lightly with the egg yolk mixture. Bake for 20 minutes or until puffed and golden brown.
*Tested in Peruvian Cooking Class