To prepare an easy filling, combine 3/4 cup of powdered sugar and one 32-ounce container of ricotta cheese in a large bowl. Stir in 1 teaspoon of vanilla and 1 cup of miniature semi-sweet chocolate chips. Chill the mixture for at least 30 minutes before using.
To make a traditional Italian pistachio cannoli filling, bring 2 cups of water to a boil in a small saucepan. Add 3/4 cup of shelled pistachios, and boil them for 1 minute. Drain and rinse the pistachios in cold water and remove the skins. Combine 1/2 cup of sugar and the pistachios in a food processor, and grind them into a powder.
In a large bowl, whisk together 5 egg yolks with 1/4 cup of sugar. Stir in the pistachio mixture.
Bring 1 3/4 cup of milk and the zest of 1 lemon to a boil in a small saucepan over medium heat. Remove the milk from the heat, and add it to the pistachio mixture, stirring constantly until they are well blended. Return the entire pistachio mixture to the saucepan, and let it cook for 2 minutes over medium heat.
Meanwhile, in a small bowl whisk together 1/4 cup of water and 3 tablespoons of cornstarch until smooth. Add the cornstarch mixture to the saucepan and stir until the liquid thickens. Stir in 1 teaspoon of vanilla.
Pour the filling into a large mixing bowl, cover it with plastic wrap and refrigerate the filling until it is cold.