Preheat oven to 350F. Line an 8-by-8-inch baking pan with aluminum foil, spray with cooking spray; set aside.
To a large, microwave-safe bowl add the butter and heat on high power to melt, about 1 minute.
Wait momentarily before adding the egg so you don’t scramble it. Add the peanut butter, egg, brown sugar, vanilla, and whisk until smooth.
Add the flour and stir until just combined, don’t overmix.
Add 1/2 cup M&Ms and stir gently to combine.
Turn batter out into prepared pan, smoothing the top lightly with a spatula.
Add the remaining 1/4 cup M&M’s to the top of the batter for visual appeal. Press them down lightly so they adhere.
Bake for about 22 to 26 minutes, or until done. A toothpick inserted in the center should come out clean, or with a few moist crumbs, but no batter.
Allow blondies to cool in pan for at least 1 hour before slicing and serving.
Notes
Peanut butter: I use Jif, Skippy or similar; natural PB can separate and/or be too oily.M&M's: I used plain milk chocolate but you can use peanut, dark, etc.Storage: Blondies are best fresh but will keep for up to 1 week in an airtight container at room temp or up to 4 months in the freezer.Adapted from Peanut Butter Blossom Cookie Bars.