Preheat oven to 350 degrees F.
In the bowl of a standing mixer, or a large mixing bowl using a hand mixer, combine the butter, oil, sugar, egg, and vanilla.
Combine the flour, salt, and baking powder in a separate bowl.
Add it in 2 increments to the butter/sugar mixture.
Put in a 9 x 9 inch prepared baking pan and press to the sides.
Sprinkle M & M's generously, pushing them gently into the dough.
Bake for 20 to 25 minutes.
Watch closely at the end. Cake should be a light, golden brown when done.
Cool completely. Cut into pieces and serve.