Preheat your oven to 350°F. Spray a 9x13-inch baking dish with nonstick cooking spray or line it with parchment paper.
In a large mixing bowl, combine (1) 15.25-ounce box of yellow cake mix, 1 cup whole milk, ½ cup sour cream, ½ cup vegetable oil, 3 large eggs, and 1 tablespoon pure vanilla extract. Beat the ingredients together until smooth, about 2 minutes. Pour the batter into the prepared baking dish and bake for 30–35 minutes or until a cake tester or toothpick inserted in the center comes out clean.
Let the cake cool for 15 minutes. Then use the handle of a wooden spoon, or a large straw to poke holes all over the top of the cake, about an inch apart. I like to use a grid-type pattern when I do mine. Let the cake cool completely to room temperature.
While you are letting the cake cool, make the cannoli filling. In a mixing bowl, stir together 1 ½ cups of drained ricotta, 1 cup of mascarpone, 1 ¼ cups of powdered sugar, 1 teaspoon of pure vanilla extract, and ½ teaspoon of ground cinnamon until the ingredients are well combined and smooth. Fold in ¼ cup mini chocolate chips. Scoop the filling into a piping bag or zip-top bag and place the cannoli filling in the fridge until your cake has cooled completely.
Once the cake is cooled, pipe the cannoli filling into each hole that you created.
Cover the filled cake and chill it for at least 1 hour in the refrigerator to let the filling firm up and for the flavors to settle.
To make the mascarpone frosting, add (2) 8-ounce tubs of cold mascarpone, 3 tablespoons heavy whipping cream, 1 tablespoon pure vanilla extract, and ¼ teaspoon kosher salt to a large mixing bowl and mix the ingredients together with a hand or stand mixer just until combined and smooth. Don’t overbeat.
Gradually mix in 4 ½ cups of powdered sugar, 1 cup at a time, beating on low. Once incorporated, beat on medium-high until fluffy. If the frosting is too thick, mix in heavy cream, 1 tablespoon at a time until smooth and spreadable.
Spread the frosting over the chilled cake. Sprinkle with ⅓ cup mini chocolate chips and 2 tablespoons of chopped pistachios (optional).
Slice into equal squares, serve cold or at room temperature. ENJOY!