Set a heatproof bowl over a pot of barely simmering water, making sure that the bottom of the bowl doesn't touch the water.
Combine the chopped chocolate and the cocoa butter and stir frequently until melted and smooth.
Use a small spoon to cover bottom and sides of egg-shaped silicone molds with a good coating of chocolate, making sure to cover any thin spots. You should use about 1/2 to 2/3 of the chocolate, saving the rest to top the eggs after filling.
Place mold on a baking sheet and freeze until set, about 20 minutes.
Beat the butter with the powdered sweetener until well combined. Slowly drizzle in the condensed milk, beating on medium low, until smooth.
Remove about 2 tablespoons of the mixture to another bowl and whisk in a little yellow food coloring at a time until a bright yellow is achieved.
Add a tablespoon or so of white condensed milk mixture to each chocolate shell, then dab a small amount of yellow condensed milk mixture in the center of each. Top with more white condensed milk.
Set the eggs in the freezer for another hour to firm up.
Re-warm the remaining chocolate gently over the pan of simmering water. Spoon over the filling and spread to the edges. Return to the fridge or freezer for another 30 minutes to firm up.