In a bowl, add brown sugar, melted (and cooled) butter, egg, and vanilla extract. Mix until well combined.
Add the dry ingredients – flour, salt, baking soda, and dark cocoa powder – and mix again until combined.
Add the chocolate buttons (or whatever you are using) and mix again.
Cut each croissant in half, horizontally. Fill the croissant with some cookie dough, and then add more on top. Repeat until all croissants are filled and topped with cookie dough. Place them on your prepared tray.