Preheat oven to 350 degrees. Line a quarter sheet pan with parchment paper. Set aside.
To a mixing bowl, add the flour, baking soda, baking powder, and salt and whisk to combine. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, combine the butter, oil, sugar, and light brown sugar. Cream together until well combined. Add the egg, egg yolk, and vanilla extract. Mix until well incorporated.
Slowly add in the flour mixture and mix until just combined. Add the M&M candies and fold them in by hand. Spread the dough evenly into the prepared quarter sheet pan. Bake for 15 minutes until a toothpick inserted comes out clean or mostly clean. Allow the bars to cool for 30 minutes before frosting.
To the bowl of a stand mixer fitted with a whisk attachment, add the butter and process until smooth. Slowly add in the powdered sugar and mix until well combined. Add the cream and vanilla extract and mix until light and fluffy.
Remove the bars from the pan and spread the frosting evenly over the top. Decorate with M&M candies and red and green sprinkles. Cut into squares or triangles.