Preheat your oven to 350°F (175°C) and line a 9x13 inch baking pan with parchment paper.

In a saucepan over medium heat, melt the butter and cook until it turns golden brown and has a nutty aroma, about 5-7 minutes. Remove from heat and let it cool slightly.

In a large mixing bowl, combine the browned butter, brown sugar, and granulated sugar. Whisk until smooth.

Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

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

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Fold in the white chocolate chips, red and blue M&M's, and chopped nuts if using.

Pour the batter into the prepared baking pan and spread it evenly.

Bake in the preheated oven for 25-30 minutes, or until the edges are golden and a toothpick inserted in the center comes out clean.

Let cool in the pan for about 10 minutes before lifting out and cutting into squares. Serve warm or at room temperature.