For filling, place sugar and cornstarch in saucepan and stir in the milk. Cook over low heat, whisking constantly, until mixture thickens and is bubbly, 5 minutes. Remove pan from heat and let mixture cool slightly. Place ricotta cheese in large bowl and blend with mixer on low speed until creamy, 1 minute. Fold in cooled milk and sugar mixture, orange zest, Marsala wine, if desired, and vanilla. Blend with mixer on low speed until well combined, 1 minute. Fold in chocolate chips. Set bowl in refrigerator, covered, while you bake the cake.
Preheat oven to 350°F Generously grease and flour two 9 inch round cake pans.
Place cake mix, milk, melted butter, eggs, and vanilla in large mixing bowl. Blend with mixer on low speed for 1 minute. Stop machine and scrape down sides of bowl. Increase mixer speed to medium, beat 2 minutes more, scraping sides down again if needed. The batter should look well combined. Divide the batter between the prepared pans. Place pans in oven.
Bake cakes until they look golden brown and spring back when lightly pressed with your finger, 27-29 minutes. Remove pans from oven and place on wire rack to cool for 10 minutes. Run a dinner knife around the edge of each layers and invert each onto wire rack. Allow to cool completely, 30 minutes more.
Meanwhile, prepare buttercream frosting.
Place butter in mixing bowl, blend with mixer on low speed until fluffy, 30 seconds. Stop machine and add confectioners' sugar, 3 T. milk, and vanilla. Blend with mixer on low speed until sugar is incorporated, 1 minute. Increase mixer speed to medium and beat until light and fluffy, 1 minute more. Blend in up to 1 T. milk if the frosting seems too stiff.
When the cake layers are cool, slice each in half horizontally using a serrated bread knife. Place the bottom of one layer, cut side up, on platter and frost with a third of the Cannoli filling. Top with the top of the first layer, cut side down, and frost with another third of Cannoli filling. Top with the bottom of the second layer, cut side up, and frost with the remaining third of the Cannoli filling. Place the top of the second layers, cut side down, on top of the filling. Spread the frosting on the top and sides of the filled cake with clean, smooth strokes. Chill at least 30 minutes before slicing and serving.