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Combine the granulated sugar, butter, vanilla, salt, cinnamon, cardamom and nutmeg in the bowl of an electric stand mixer fitted with the paddle attachment. Beat on medium speed until light and fluffy, 3 to 5 minutes. Turn the speed to low and add the eggs one at a time; beat until fully incorporated. Mix in the cocoa powder. Stop the mixer and scrape down the bowl to ensure that all of the ingredients have been fully incorporated. Add the baking soda and 1 1/2 cups of the flour and mix on low just until combined. Slowly drizzle in the buttermilk. When the buttermilk is almost fully incorporated, add the remaining 1 1/2 cups of flour. Add the boiling water and mix just until the batter is smooth.
Evenly distribute the batter into the prepared pans and bake for 25 minutes. Rotate the pans and bake until a toothpick or cake tester inserted in the center comes out clean, an additional 10 minutes. Cool the cakes in the pans on a wire rack for at least 2 hours.
For the chocolate spice buttercream frosting: Combine the butter, cocoa powder, vanilla, salt, cinnamon, nutmeg, cardamom and 6 cups of the confectioners' sugar in the bowl of an electric stand mixer fitted with the paddle attachment. Add 1/2 cup of the milk and mix on low speed until combined, about 1 minute. Add the remaining 6 cups confectioners' sugar and 1/4 cup milk and mix until fully combined, 1 to 2 minutes more. If the frosting seems too thick to spread, slowly drizzle in a little bit more milk, about 1 teaspoon at a time, to achieve the desired consistency. Turn the mixer to high and beat the frosting until light and fluffy, about 3 minutes.
To assemble: Remove the cakes from the pans. Using a large serrated knife, trim the cakes so that they are as flat as possible. Place one of the layers on a plate or cake stand and pipe or spread with an even 1/2 inch of frosting. Repeat with the remaining layers. Chill the cake in the refrigerator or freezer for 30 to 60 minutes.
Frost the outside of the cake with the remaining frosting, and with a small, offset spatula or butter knife, create a swirl pattern in the frosting. Garnish with sprinkles if using. Chill the cake in the refrigerator for 1 hour to set the frosting.
Serve the cake at room temperature. Since this is a butter-based cake, store it at room temperature and not in the refrigerator to ensure it doesn't dry out.