Preheat oven to 350°F / 177°C. Line an 8x8 metal baking pan with parchment paper and / or spray the pan with nonstick cooking spray.
In a medium mixing bowl, whisk together the flour, baking powder, ground cinnamon, and salt. Set aside.
In a larger mixing bowl, whisk together melted butter and brown sugar until evenly combined. Whisk in egg and vanilla.
Pour dry ingredients over wet and stir with a spatula or wooden spoon just until no dry spots remain (do not overmix.)
Gently stir in oats, chocolate chips, and M&Ms.
Pour batter into prepared pan, scattering any reserved mix-ins over top.
Bake 24 to 28 minutes, until the edges are lightly browned. (Note: If cooked on the lower end of the bake time, these will be very soft and will stay moist for a couple of days. If you plan to serve them the same day as they were baked, bake at the higher end of the recommended cook time.)
Let cool in the pan until the bars can be easily removed. Once cool, slice and serve.
The bars can be stored at room temperature for up to 3 days.