Preheat your oven to 350°F (175°C).
Line a 9x9-inch baking pan with parchment paper, allowing some overhang on the sides for easy removal later. Lightly grease the parchment paper.
In a medium saucepan, melt the butter over low heat. Once melted, remove the saucepan from heat and let it cool slightly.
In a large mixing bowl, combine the melted butter, dark brown sugar, and granulated sugar. Whisk until the mixture is smooth and well combined.
Add in the egg and vanilla extract to the sugar mixture. Whisk until fully incorporated.
In another bowl, whisk together the flour and kosher salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix!
Gently fold in the sweetened flaked coconut and mini M&Ms until evenly distributed throughout the batter.
Pour the batter into the prepared baking pan, spreading it evenly. Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with a few moist crumbs.
Allow the blondies to cool in the pan for about 10 minutes before lifting them out using the parchment overhang. Let them cool completely on a wire rack before cutting into squares.
