Heat oven to 350 degrees. Generously butter the inside of two 8-inch Bundt pans; dust lightly with cocoa powder.
In the bowl of an electric mixer, combine granulated sugar, flour, cocoa, baking soda, baking powder, and salt, and set aside.
In a medium bowl, whisk together eggs, vanilla, canola oil, buttermilk, and 3/4 cup milk. Slowly add egg mixture to sugar mixture. Using the paddle attachment, mix batter on low speed until smooth, about 1 minute. Divide batter evenly between the prepared pans. Bake cakes until a cake tester inserted into the center comes out clean, about 40 minutes. Transfer to a cooling rack, and cool for 20 minutes before removing from the pans.
Place both chocolates, heavy cream, and 1 tablespoon butter in a medium heat-proof bowl over a pot of gently simmering water. Heat, stirring often, until chocolate is completely melted, about 5 minutes. Remove from the heat, and let cool to room temperature. Whisk chocolate until lightened; set aside.
Using a serrated knife, trim the bottoms of both cakes so they are both flat. Spread chocolate evenly on the flat side of one of the cakes. Invert the remaining cake onto chocolate so that a pumpkin shape is formed. Place cake onto a wire rack.
To make orange butter glaze, combine remaining 1/4 cup plus 1 tablespoon of milk and the food coloring until the mixture is a pumpkin color; set aside. In a medium bowl, whisk together confectioners' sugar and 1 cup melted butter. Add reserved milk mixture, and continue whisking until smooth. Working quickly, carefully pour orange butter glaze over the cake, tilting the cake as necessary to coat all sides of the cake. Let glaze set completely before garnishing with chocolate leaves. Serve.
To make a stem, twist a brown-paper lunch bag into a stem shape, and wrap it tightly with green floral tape. Insert the stem into the center of the cake. Remove before serving.