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1
Preheat oven to 350 F. Grease three 9-inch round cake pans, and line the bottoms with parchment paper.
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2
In a large bowl, whisk the flour, baking powder, baking soda and salt. Set aside.
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3
In another bowl, whisk the boiling water and cocoa powder until smooth. Set aside.
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4
Beat butter and sugar together on medium speed in the bowl of a standing mixer until light and fluffy. Beat in eggs, one at a time. Beat in buttermilk and vanilla extract.
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5
Reduce the speed to low and beat in 1/3 of the flour mixture, followed by half of the chocolate mixture. Repeat with another 1/3 of the flour mixture and the remaining chocolate mixture. Beat in the remaining flour mixture until just incorporated.
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6
Divide the batter in thirds and pour into the prepared pans, smoothing the surface with a spatula. Put pans into the oven and bake 20 to 25 minutes or until a toothpick inserted into the cake comes out clean.
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7
Place the cakes (in the pans) on a wire rack to cool. Then invert the cakes onto a greased rack. Cool completely before frosting.
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8
To prepare the frosting, melt chocolate chips and cream in the microwave. Heat for 30 second increments, stirring in between and continuing only until it's smooth. Set aside.
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9
In a standing mixer with a paddle attachment, beat butter for a few minutes on medium speed. Add cooca powder and powdered sugar and beat until incorporated.
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10
Add melted chocolate and rum, and beat until you get to the right consistency.
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11
To assemble the cake, place one cake layer on your serving plate and spread the top with 1/3 cup frosting. Place another cake layer on top, and gently press down a little bit. Spread with another 1/3 cup frosting. Place the last layer of cake on top, smear the remaining frosting on the top and sides of the cake.
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12
Cover and refrigerate. Remove from refrigerator an hour before serving.
