Preheat oven to 350F with rack on lower middle position. Line 8x8 baking pan with parchment paper or foil, with enough excess to create a "sling" that reaches over the edges of pan for ease of dessert removal. Lightly grease the paper or foil. Set aside.
In bowl of stand mixer fitted with paddle attachment (or you can use a large mixing bowl and electric hand mixer), add the softened butter and brown sugar. Mix on medium high until mixture is fluffy.
Add vanilla and egg, mixing to combine. Slowly and gently add the flour and salt, switching to low speed; mix just until combined - do not over mix.
Using a rubber spatula, gently fold in the chocolate morsels and M&M's. Scrape dough into lined/greased pan, ensuring corners are filled evenly.
Bake about 25 min or just until toothpick inserted in center comes out barely clean. A few tender crumbs attached are great.
Let cool completely in pan on wire rack. Carefully remove entire dessert with sling overhang, directly onto a cutting board. Cut into bars and plate to serve.
Notes
Loosely cover dessert with foil if top browns too quickly during baking.
Chewy blondies keep well in an airtight container at moderate room temp for several days.
Feel free to experiment with different types of chocolate or M&M's to suit the occasion. See original article for ideas.
If you enjoyed this recipe, please come back and give it a rating. We ❤️ hearing from you! Join our Free Recipe Club and get our newest, best recipes each week!