Preheat your oven to 350°F (175°C). This will ensure your blondies bake evenly.
Line a 9x9 inch baking pan with parchment paper, allowing some overhang for easy removal later.
In a large mixing bowl, combine the melted butter, peanut butter, and brown sugar. Whisk until smooth and creamy. Add the egg and vanilla extract, mixing well until fully incorporated.
Gently fold in the all-purpose flour until just combined. Be careful not to overmix.
Reserve about 1/4 cup of M&M's for topping and fold the rest into the batter.
Pour the batter into the prepared baking pan, spreading it evenly. Sprinkle the reserved M&M's on top.
Bake in the preheated oven for about 25-30 minutes, or until the edges are golden and a toothpick inserted in 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 paper. Let them cool completely on a wire rack before slicing into squares.
