Preheat oven to 325 degrees. Invert two 9 1/2-by-3-inch disposable round foil baking pans. Using a craft knife or a paring knife, cut six X shapes in bottom of each, spacing them evenly, to make 1 1/2-inch openings. Fit a waffle cone snugly into each opening, standing the cones upright. Line each cone with parchment paper, and then place a dried bean or a hazelnut in bottom to prevent batter from leaking. Place pans and cones on parchment-lined baking sheets. Line a 12-cup mini-muffin tin with paper liners.
Whisk together cocoa powder, flour, sugar, baking soda, baking powder, and salt in a large bowl. Whisk together eggs, warm water, buttermilk, oil, and vanilla in a medium bowl. With a mixer on low speed, gradually add egg mixture to flour mixture, beating until just combined. Raise speed to medium, and beat for 3 minutes. Carefully pour batter into cones, filling each two-thirds full. Spoon 1 1/2 tablespoons batter into each muffin cup (these will be the tree bases).
Bake until a skewer inserted into center of cake comes out clean, 15 to 18 minutes for cupcakes and 35 minutes for cones. Let cakes cool on a wire rack for 15 minutes. Remove cakes from cones and parchment. Let cool completely on the rack. (Cakes can be wrapped in plastic and stored at room temperature for up to 3 days before assembling.)
If necessary, use a serrated knife to trim top of each cupcake to create a flat surface. Place a dab of frosting on top of each cupcake, and position a cone cake on top. Insert toothpick through each cupcake base and into "tree" to support it (remember to remove toothpick before eating). Place frosting in a pastry bag fitted with a small star tip (such as Ateco #3) or a small plain 1/8-inch tip, and pipe frosting decoratively all over tree. Sprinkle with sanding sugar. Trees can be refrigerated, uncovered, for up to 6 hours before serving.