Place the butter and sugar in a bowl and beat with an electric mixer until light and fluffy. Add 1 egg at a time, mixing well after each addition. Then add the vanilla extract and combine.
Sift together the flour, baking powder and salt. Add half of the flour mixture, then the buttermilk, then the remaining flour mixture. Pour 1/3 of the mixture into a separate bowl.
In a separate bowl, combine the cocoa powder and boiling water. Add this to the smaller amount of cake batter. Mix well, then add some black food coloring and mix until combined.
Spoon the cake batter into a buttered 9-inch round cake pan, making a checkerboard pattern with the chocolate and vanilla batter.
Bake at 350F for 40-50 minutes. Transfer the pan to a cooling rack and cool for 5 minutes. Then turn the cake out from the pan and let fully cool. In the meantime, make the two buttercreams.
To make the white buttercream: Cream the butter in a large mixing bowl until it's light and fluffy. Add the vanilla extract, and mix together. Add the cream of tartar and confectioner's sugar 1 cup at a time, beating with each addition.
Place 1/3 cup white buttercream in a small bowl and add some pink food coloring. This will be used for the cow's nose and ears.
To make the black buttercream: Cream the butter in a large mixing bowl until it's light and fluffy. Add the confectioner's sugar 1 cup at a time, beating with each addition. Add the milk and salt and mix for about 3 minutes.
Add the melted chocolate and beat for about 1 minute.
Assembly: Place a plate face down on top of the cake and use it as a guide to trim about 1 inch from all sides of the cake. Carve little ears and horns from the cake scraps.
Slice the cake in half to create 2 layers. Place the bottom layer of the cake on a serving dish and spread some white buttercream onto the surface. Sprinkle with the blackberries, cherries and mini chocolate chips. Place the second layer of cake on top and gently press down.
Coat the entire surface of the cake in a thin layer of white buttercream. Attach the ears and horns with toothpicks.
Place the remaining white buttercream, pink buttercream and black buttercream in piping bags fitted with star-shaped tips.
Pipe the pink nose onto the cake, then pipe the details with the black buttercream.
Use the remaining black buttercream to pipe the eyes and spots. Pipe pink buttercream onto the center of the ears.
Fill in the rest of the surface of the cake with white buttercream, including the horns.