Combine condensed milk, cocoa powder, and butter in a pot. Bring to a boil over medium heat, stirring constantly, and cook until mixture begins to pull away from the sides and bottom of the pan. Remove from heat and stir in cream. Beat well with a whisk until smooth. Add more cream, 1 tablespoon at a time, if the filling is too firm.
Place milk chocolate in top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is about 80% melted, about 5 minutes.
Pour chocolate onto a clean, smooth surface, ideally marble or granite, and use a knife or spatula to spread and temper. Test the temperature of the chocolate by bringing a chocolate-dipped toothpick to your lips; the chocolate must feel cold on the lips. Continue cooling and testing if it's too hot. Transfer tempered chocolate to a bowl.
Spread 1 layer of chocolate evenly in the Easter egg mold halves and refrigerate for 10 minutes. Remove from the refrigerator and add another layer of chocolate to each egg half. Fill chocolate eggs with brigadeiro filling almost up to the edges. Seal filling with another layer of melted chocolate, making sure the filling is completely covered with chocolate and nothing can leak out. Chill until firm and set, about 20 minutes.
Unmold chocolate eggs one at a time by placing them upside down on a clean surface. Spread a thin layer of melted chocolate on the outside and cover with chocolate sprinkles.
