Bake two 8-inch round cakes according to the package directions. Cool completely. Cut a parchment sheet into four 3-by-12-inch strips and set aside.
Meanwhile, combine the butter, sugar and salt in the bowl of a stand mixer fitted with a paddle attachment. Beat on the lowest speed until the sugar is incorporated into the butter. Add the vanilla, increase speed to medium-high and beat until smooth, about 2 minutes. With the mixer running on medium high, add the milk, 1 tablespoon at a time, until the frosting is smooth and spreadable.
To make the filling, remove 2 cups of the frosting and place in a medium bowl. Add the cinnamon and mini chocolate chips and stir until combined. Reserve remaining frosting.
Using a serrated knife, trim the top of each cake to make level and straight. Line the edges of a cake stand with the parchment strips (to keep it clean when you frost and glaze the cake with chocolate). Place one cake in the center of the cake stand. Using a small offset spatula, spread the chocolate filling evenly over the top of the cake. Top with the second cake.
With a large offset spatula, spread 3/4 cup of the reserved frosting in a thin layer on the top and sides of the cake. This is called a crumb coat. Place the cake in the freezer until the frosting is set, about 30 minutes.
Once chilled, remove the cake from the freezer and spread with all but 1/2 cup of the remaining frosting. Smooth the sides and top of the cake using the offset spatula or a bench scraper. Return to the freezer for another 30 minutes to set the frosting.
Meanwhile, make the ganache by placing the semi-sweet chocolate chips in a small heatproof bowl. Heat the cream until hot but not boiling. Pour over the chocolate and let sit for 5 minutes, then stir until smooth and glossy.
Remove the cake from the freezer and pour the ganache over the top. Use the back of a spoon to completely cover the top, allowing some of the chocolate to spill over the sides in a drip pattern.
Place the remaining 1/2 cup frosting in a pastry bag fitted with a medium star tip. Pipe rosettes on the chocolate around the top edge of the cake, spacing them about 1/2 inch apart. Place one end of a cannoli on each rosette, so the opposite ends meet in the center of the cake. Remove the parchment strips from beneath the cake and serve.
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