Divide the chopped chocolate into three equal parts.

Combine 2 parts in a glass (microwaveable) bowl and bring it to the microwave to melt on high in 20 to 30 second bursts, stirring gently, until it is melted and the thermometer reads in between 114 to 118F degrees or 108 to 113F degrees if using only milk chocolate. (You may need to microwave in 5-second bursts toward the end.)

Then, slowly add the remaining chocolate to the melted chocolate, stirring gently, until all the chocolate is melted and the thermometer reads 88 to 89F degrees (85 to 86F degrees of using only milk chocolate).

Using a silicone brush, brush a layer of chocolate into your mold(s). Bring the mold(s) to the fridge for 10 minutes. Then, brush another layer and bring it to the fridge again, for 10 more minutes. Repeat this process until you've brushed 4 or 5 layers of chocolate and your egg(s) is/are firm and thick enough to hold the brigadeiro, leaving the mold(s) in the fridge for 15 minutes after the last layer.

When the chocolate is solid, use a butter knife to carve out the excess and make the edges neat. Then, flex the mold(s) to gently release the chocolate egg)s). Take your time - you will see the air slowly creep its way between the plastic and the shiny, hard chocolate. If you're having difficulty releasing the chocolate egg(s) from the mold(s), just bring it back to the fridge, upside down, with some parchment paper under it, and the egg(s) will eventually release by itself.

In a saucepan, combine the butter, sweet condensed milk and the cocoa powder. Bring it to the stove, over medium low heat, stirring until everything is combined. Keep stirring (constantly!) and cook until the brigadeiro thickens and starts releasing from the bottom of the pan (about 10 minutes).

Remove from heat and add the heavy cream, mixing until smooth. Then, transfer the brigadeiro to a medium sized bowl and wait for it to cool completely before adding to the chocolate egg.

When cool, fill the chocolate egg(s) with the brigadeiro and finish by decorating it with the chocolate sprinkles.