For Chocolate Buttercream Frosting: In the bowl of an electric mixer, blend butter, shortening and sugar with the paddle attachment until creamy. Add the cocoa to the butter mixture along with the milk and extract. Continue to mix until well combined. Turn off the mixer and gently fold in chocolate chips with a spatula. Set frosting in the refrigerator to firm up to a spreadable consistency while the cake is prepared.
Preheat oven to 350 degrees F. Coat 2 (9-inch) round cake pans with nonstick cooking spray, including sides. Cut a sheet of parchment paper to fit the pan bottoms and lay in place. Then spray the paper.
Sift together the flour, baking powder and salt, set aside. In a large mixing bowl, cream the butter until smooth. Gradually add the sugar and beat until light and fluffy. Beat in the eggs, a little at a time. Stir in the vanilla until well blended. Scrape down the sides of the bowl. Gradually mix in the dry ingredients in 3 batches, alternating with the milk. Beat for 1 minute after each addition to incorporate the ingredients and strengthen the cake's structure. Mix until the batter is smooth.
Pour the batter into the prepared pans and smooth the surface with a spatula, the pans should be 2/3 full. Place pans on the middle rack of the oven and bake for 20 to 25 minutes, or until a knife inserted in the center comes out clean and the cake springs back when touched.
Let the cakes cool in the pans for 10 minutes, then turn them out onto a wire rack, peel off the paper and let cool for at least 1 hour. Level the tops of the cakes with a serrated knife if domed in the middle. To divide each cake in half to give 4 layers: cut a vertical notch on the side of the cakes so you can easily line up the layers when assembling. Place one hand flat on top and the serrated blade along the side of the cake. Rotate the cake against the knife as you slice. Carefully separate the layers and brush away any loose crumbs. To make sugar glaze: combine sugar, water and rum in a small saucepan. Boil for 2 to 3 minutes. Brush the sugar glaze on the cake layers to moisten and add extra flavor.
Put a cardboard cake round or plate on a lazy Susan; so it easier to turn and frost the cake. Lay 1 cake layer on the plate. With a metal spatula, spread about 1/2 cup of chocolate buttercream frosting on top of the base. Carefully place another cake layer on top. Do this for the remaining 2 layers.
Frost the top and sides of the cake thoroughly. Refrigerate for 45 minutes before decorating or cutting.
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