Make the nest: Preheat oven to 350 degrees. Lightly brush a 1/3-cup-capacity round silicone mold with butter. In a bowl, gently toss butter and kataifi to combine.
Lift kataifi out of bowl, gently stretching strands to form a loose rope. Lay 1 end in the cupped fingers of 1 hand. Gently spiral rope to shape into a nest.
Fit nest into mold, pulling up edges to rest over rim. Place on a baking sheet. Bake until golden brown, 25 to 30 minutes. Let cool completely in mold on a wire rack. Unmold. Dust with cocoa.
Make the cake: Butter two 10-by-2-inch round cake pans. Line with parchment cut to fit, and butter parchment. Dust with cocoa, and tap out excess. Sift flour, sugar, cocoa, baking soda, baking powder, and salt into the bowl of a mixer. Add remaining ingredients. Mix on low speed until smooth, about 3 minutes. Divide batter among prepared pans.
Bake, rotating pans halfway through, until a toothpick inserted into centers of cakes comes out clean, about 35 minutes. Let cool in pans on wire racks for 45 minutes. Run a knife around edges of cakes to loosen, and turn onto racks. Remove parchment, turn cakes right side up, and let cool completely.
Make the frosting: Sift confectioners' sugar, cocoa, and salt into a bowl. In the bowl of a mixer, beat cream cheese and butter on medium-high speed until fluffy, about 4 minutes. Reduce speed to low. Gradually add sugar mixture, beating until combined. Beat in melted chocolate and sour cream until combined.
Make the garnish: Using a vegetable peeler, scrape chocolate at a 45-degree angle. Using a spatula, transfer curls to a parchment-lined baking sheet. Cover with plastic wrap, and refrigerate for at least 30 minutes (or up to 4 days).
To assemble: Using a serrated knife, trim tops of cakes to create level surfaces. Reserve trimmings. Place 1 cake, cut side up, on a cake stand. Spread 2 cups frosting over top. Top with remaining cake, cut side up. Spread remaining frosting over top and sides. Finely grind cake trimmings in a food processor. Sprinkle crumbs over top of cake.
Using tweezers, gently press chocolate curls to sides of cake. Dust top with confectioners' sugar if desired. Decorate with nest, chocolate eggs, and flowers.
For the directions for the crepe-paper flowers, see the Crepe-Paper Crocuses and Snowdrops How-To.