Beat softened butter and brown sugar until smooth and fluffy.
Add vanilla and mix until combined.
Gradually add cooled flour and sweetened condensed milk, alternating between the two, until a soft dough forms.
Fold in chocolate chips and mini M&M’s.
Press dough evenly into the prepared pan.
Melt peanut butter and milk chocolate chips together until smooth.
Spread topping over the cookie dough layer.
Chill until firm, then slice into bars.
Notes
Heat-treating flour makes the cookie dough safe to eat uncooked. Let flour cool before adding it so the butter does not melt. Let bars sit at room temperature for a few minutes before slicing for cleaner cuts. Store covered in the refrigerator or freezer for best texture.