Making brigadeiros in Portugal similar to the ones in Brazil
I never use condensed milk that I haven’t made myself. Just take heavy cream, slowly whisk in 1:1 (by weight) confectioners sugar and simmer for three hours - stirring every 10 minutes. I would also be concerned by the butter you are using. I would melt it in a separate saucepan and filter it through a coffee filter to remove casein solids, whisk in your cocoa and then stir the milk/sugar and butter/cocoa components together.

Depending on your desired consistency (and shine) you could also add honey or sugar syrup...