Using a 2-pound plastic Easter egg chocolate candy mold as a guide, cut the cake to fit inside the base of the mold; set aside (discard the extra cake). Refrigerate the mold to chill, about 15 minutes.
Put half the milk chocolate in a microwave-safe bowl and microwave in 30-second intervals, stirring after each, until melted, about 3 minutes. Add the remaining chocolate and continue to stir until smooth; this will result in a tempered chocolate that will set hard. Transfer 3/4 cup of the chocolate to a small bowl; set aside.
Swirl half the remaining chocolate inside the mold to coat completely. Refrigerate until set, about 15 minutes. Repeat with the other half of the chocolate, swirling to create a thick layer; refrigerate for 15 minutes.
Fill the egg with the candy. Top with the cake cutout and press down firmly so the cake is flush with the mold (trim away any extra cake with a serrated knife).
Microwave the reserved chocolate to remelt if needed, then measure out 1/2 cup. Spread the chocolate over the cake with a spatula to cover completely. Refrigerate until set, at least 30 minutes.
Invert the egg onto a serving platter or cake stand; remove the mold. Microwave the remaining chocolate to remelt if needed, then transfer to a piping bag with a small round tip. Use the chocolate to attach candies to the outside of the egg. Chill until ready to serve.
Serve the egg with a big spoon or meat mallet and encourage guests to crack it open and watch the candy pour out.