Preheat your oven to 350°F (175°C). This ensures even baking of the brownies.

Grease a 9x13 inch baking pan or line it with parchment paper for easy removal.

In a large mixing bowl, combine the melted butter and granulated sugar. Whisk until the mixture is smooth and glossy, about 1-2 minutes.

Add the eggs one at a time, whisking well after each addition. This helps to incorporate air for a fudgy texture.

Stir in the vanilla extract.

In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix to keep the brownies fudgy.

Fold in the oats, chocolate chips, M&M's, and peanut butter until evenly distributed. The peanut butter adds richness and a hint of saltiness.

Pour the brownie batter into the prepared baking pan and spread it evenly with a spatula.

Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. This indicates that the brownies are fudgy and not overbaked.

Once baked, remove from the oven and let the brownies cool in the pan for at least 15 minutes before transferring to a wire rack to cool completely.

Once cooled, cut the brownies into squares and serve. Enjoy the rich, fudgy texture combined with the chewy oats and crunchy M&M's!