Preheat oven to 325°F
Line a 15" x 10" cookie sheet with parchment paper, allowing parchment paper to hang over the sides about an inch on each side
Beat melted butter and sugars together, until smooth and creamy
Beat in eggs and vanilla
Add in baking soda
Using low speed, beat in flour, ½ cup at a time, until all flour is just incorporated
Fold in 1 cup of M&Ms and chocolate chips.
Spread batter evenly on cookie sheet.
Sprinkle remaining ¼ cup M&Ms over the top of the batter
Bake for 25 minutes or until edges are browned. Centre will still be soft
Allow to cool in the pan for 15 minutes
Remove from pan using parchment paper to help lift the blondies out
Cut into 35 squares and serve.
Store leftovers in an airtight container at room temperature.