In a non-stick pan, add the condensed milk, sugar, milk, butter and vanilla extract.
Stir over low heat until the mixture reaches 230°F (or forms a soft ball when adding a splash of cold water).
Remove from heat and beat in a mixer until the cream thickens and starts to get grainy.
Add 3/4 of the M&M's and mix.
Arrange in a greased pan lined with parchment paper.
Place the rest of M&M's on top.
Let it cool.
When cold, cut into squares.