In a large bowl, mix butter with powdered sugar and vanilla extract until a thick dough forms. Fold in the M&Ms.
Roll the mixture into balls and set onto a plate. Freeze for at least 20 minutes. Place the chocolate chips into a microwave-safe bowl, and heat in the microwave in 30-second intervals, stirring well until it is fully melted and smooth.
Fully coat each truffle in the melted chocolate and set onto a plate. Let sit until the chocolate has completely set.