Mix oats through butter.
Keep aside 1 1/2 c, and press the rest into a greased 20x 30 pan.
Bake at 350°F for 10 minutes.
Meanwhile, melt condensed milk and chocolate (or 1 c M&Ms).
Pour over the base, crumble remaining mix on top, together with remaining M&Ms.
Bake for a further 20 minutes.
Cut into bars.