Lightly grease a 9 x 13-inch pan and preheat your oven to 350°F. Melt the butter in a medium-sized saucepan. Add the brown sugar, and stir until the mixture is smooth.

Remove the pan from the heat, pour the butter-sugar mixture into a medium-sized bowl, and allow it to cool to lukewarm. Whisk in the eggs one at a time. Stir in the vanilla extract and salt.

Whisk the flour and baking powder together, then add the dry ingredients to the sugar mixture, stirring to combine. Finally, fold in the M&M's and nuts.

Spread the mixture evenly in the pan, and bake the brownies for 30 to 35 minutes, until they're light brown on the edges and top