Preheat your oven to 350°F (175°C). Line your baking sheets with parchment paper or silicone mats. Separate your M&M's into two groups—one for chopping and one to keep whole.
In a large mixing bowl, use an electric mixer to cream together the softened butter, vegetable oil, granulated sugar, and brown sugar until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Then mix in the vanilla extract.
In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, beating on low speed or folding with a spatula until just combined. Avoid overmixing.
Gently fold in the chopped M&M's, reserving the whole pieces for topping.
Using a cookie scoop or tablespoon, drop dough balls onto the prepared baking sheets. Press a few whole M&M's on top of each cookie.
Bake for 10-12 minutes, or until edges are golden but centers still look slightly soft. Let cookies cool on the baking sheet for 5 minutes, then transfer to a cooling rack.