Preheat oven to 350°F (177°C). Line an 8×8 baking pan with parchment paper or grease it.
In a medium bowl, whisk together flour, baking powder, cinnamon, and salt.
In a larger bowl, whisk melted butter and brown sugar until combined, then add egg and vanilla, mixing well.
Add dry ingredients to wet ingredients and stir until just combined, avoiding overmixing.
Gently fold in oats, chocolate chips, and M&Ms.
Pour batter into prepared pan and scatter extra mix-ins on top if desired.
Bake 24 to 28 minutes until edges are lightly browned. For softer bars, use shorter bake time.
Cool bars in pan until firm enough to slice, then cut and serve.
Store bars at room temperature for up to 3 days.