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1
Put oven rack in middle position and preheat oven to 350F.
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2
Butter 2 (9- by 2-inch) round cake pans and line bottom of each with a round of wax paper.
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3
Butter paper and dust pans with flour, knocking out excess.
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4
Whisk together hot water and cocoa powder in a bowl until smooth, then whisk in milk and vanilla.
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5
Whisk together flour, baking soda, and salt in another bowl.
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6
Beat together butter and brown sugar in a large bowl with an electric mixer at medium-high speed until pale and fluffy, 3 to 5 minutes.
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7
Add eggs 1 at a time, beating well after each addition.
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8
Reduce speed to low and add flour and cocoa mixtures alternately in batches, beginning and ending with flour mixture (batter may look curdled).
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9
Divide batter between cake pans, smoothing tops.
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10
Bake until a wooden pick or skewer comes out clean and edges of cake begin to pull away from sides of pans, 25 to 35 minutes total.
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11
Cool layers in pans on racks 10 minutes, then invert onto racks, removing wax paper, and cool completely.
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12
Put 1 cake layer, rounded side up, on a cake stand or platter and, using offset spatula, spread top with about 1 cup buttercream.
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13
Top with remaining cake layer, rounded side down, and frost side and top of cake with 2 cups buttercream.
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14
For each color (you can make up to 5), transfer 3 tablespoons buttercream to a separate small bowl and tint with food coloring.
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15
Snip off 1 corner of each plastic bag to create a 1/4-inch opening, then spoon each color of buttercream into a bag, pressing out excess air.
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16
Twist each bag firmly just above buttercream, then decoratively pipe colored buttercream onto cake.
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17
Chill cake until buttercream is set, about 30 minutes.
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18
Just before serving, arrange 6 to 8 mini cupcakes on top of cake in 1 layer, then stack remaining mini cupcakes on top.
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19
*Wilton Paste Food Coloring available at wilton.com.
