In a large bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Add eggs, 1 at a time, beating well after each addition. Combine the flour, cocoa, baking powder and salt; gradually add to creamed mixture and mix well. , Spread into a greased 9-in. springform pan. Bake at 350° for 15-20 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes. Carefully run a knife around edge of pan to loosen. Remove sides and bottom of pan. Cool completely on a wire rack., From a large sheet of waxed paper, cut a 29x5-in. strip. Fold strip lengthwise in half. Place strip on a large sheet of waxed paper on a work surface. Spread melted chocolate evenly along 1 long edge of waxed paper strip; spread upward, making a wavy line to within 1/2 in. of other long edge. Let stand for 10-30 minutes until chocolate begins to set but is still pliable. , Carefully lift waxed paper strip and wrap chocolate strip around cake with straight edge on the bottom. Do not remove waxed paper. Refrigerate until chilled. Meanwhile, cover 3 baking sheets with waxed paper. Drizzle remaining melted chocolate from a spatula over waxed paper in both directions. Chill for 10 minutes. Peel off waxed paper; break chocolate into 2- to 3-in. "twigs". , For filling, in a small saucepan, combine the sugar and cornstarch. Stir in milk until smooth. Cook and stir over medium-high heat until slightly thickened and bubbly. Reduce heat; cook and stir 2 minutes longer. Remove from the heat. Stir a small amount of hot filling into egg yolks; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Remove from the heat. , In a small bowl, sprinkle gelatin over water; let stand for 1 minute. Stir into custard until gelatin is dissolved. Add chopped chocolate; stir until smooth. Cover surface with plastic wrap; cool to room temperature., In a large bowl, beat cream until soft peaks form; fold into chocolate mixture. Carefully spoon over top of cake. Cover and refrigerate for 1-2 hours or until set. Remove waxed paper strip from side of cake. Position chocolate "twigs" around top of cake to create a nest. Arrange candy eggs in center.