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

In a medium saucepan, melt the butter over medium heat. Continue cooking the butter, stirring frequently, until it turns golden brown and has a nutty aroma. Remove from heat and let cool slightly.

In a large mixing bowl, combine the browned butter, brown sugar, and granulated sugar. Mix until well combined.

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

In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Fold in the chocolate chips, 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.

Allow the blondies to cool in the pan for at least 15 minutes before lifting them out using the parchment paper. Cut into squares and serve.