Preheat the oven to 350 degrees Fahrenheit and spray a 13"x9" cake pan with nonstick baking spray.
In the bowl of an electric mixer, cream together softened butter, brown sugar, granulated sugar, eggs, vanilla extract and sour cream. Mix well and scrape down the sides of the bowl in between.
Add in melted chocolate and mix until combined.
In a separate bowl, whisk together flour, baking cocoa, salt and baking soda.
Add flour mixture to wet mixture and mix just until combined, scraping down the sides in between.
Spread evenly into prepared pan.
Sprinkle with M&M's and lightly press them into the batter.
Bake for approximately 19-20 minutes, or just until set. Be careful not to over bake.
Allow the bars to cool completely, then slice into squares and either serve immediately or store in an air tight container until ready to serve.