Preheat the oven to 350 degrees. Heat the olive oil in a skillet over medium heat. Add the pancetta and fry until crisp, then drain on paper towel. Whisk the eggs with the heavy cream and plenty of salt and pepper. Add the parsley, chives, pancetta and Parmesan and mix well. Butter the muffin pan, and then ladle the mixture into each hole, two-thirds full (there will be batter left-over to make another batch). Bake until firm, 20 to 25 minutes. Remove the frittatas from the pan and let them cool on a wire rack while you bake the rest. These can be made up to four hours in advance. To serve, you can re-heat the frittatas loosely wrapped in foil, in a 350-degree oven for about 10 minutes. Frittatas can be eaten hot, warm or cold.