Cake: Preheat oven to 350 degrees. Butter two 8-inch round cake pans; dust with cocoa. Whisk together cocoa, flour, granulated sugar, baking soda, baking powder and salt; beat on low speed until just combined. Increase speed to medium and add eggs, buttermilk, water, oil, and vanilla. Beat until smooth, about 3 minutes.

Divide batter between prepared pans. Bake until set and a toothpick inserted into centers comes out clean, about 35 minutes. Let cool 15 minutes, then turn out from pans and transfer to wire racks. Let cool completely.

Buttercream: Meanwhile, beat butter on medium-high speed until pale and creamy, about 2 minutes. Reduce speed to medium and add confectioners' sugar, 1/2 cup at a time, beating after each addition. (After every two additions, increase speed to high and beat 10 seconds, then reduce speed to medium again.) Add vanilla and beat until buttercream is smooth. Use immediately, or cover and refrigerate up to 3 days. Bring to room temperature, and beat on low speed until smooth before using.

Nest: Pour vodka into an 8-inch cake pan. Freeze until very cold, at least 30 minutes. Meanwhile, melt chocolate in a heatproof bowl set over (but not in) a pot of boiling water. Let cool slightly, then transfer to a piping bag; snip off a very small piece of end. Drizzle chocolate into cold vodka in a circular motion, stopping after a few rings form. Remove chocolate rings with two forks to a paper towel-lined plate. Repeat with remaining chocolate, layering rings to build a nest shape. Refrigerate until nest is firm enough to lift and condensation has evaporated, about 30 minutes.

Spread 2 cups frosting onto top of 1 cooled layer. Top with remaining layer; frost top and sides with remaining 2 cups frosting. (Unfrosted cake layers can be stored, wrapped in plastic, at room temperature or refrigerated overnight. Frosted cake can be refrigerated overnight.) Place nest atop cake, and nestle in candies and bird.