Combine the sweetened condensed milk, pumpkin puree, and butter in a small saucepan. Cook over medium heat, stirring constantly until it starts to boil.
Continue cooking for about 15 minutes until the mixture is very thick. If you scrape the bottom of the pan with a spatula, the brigadeiro should take about 5 seconds to come back together.
Pour the mixture onto a plate and let it cool. Chill it in the fridge for at least 1 hour until completely cold.
Butter your hands, scoop a small amount of the mixture, and roll it into a ball. Use a toothpick to make dents to form a pumpkin shape. Place a mini chocolate chip on top as the pumpkin stem.