I a 2-3 quart pan, melt butter and chocolate over medium low heat, stirring well until blended. Remove from heat and stir in sugar and vanilla. Add eggs one at a time, beating well after each addition. stir in flour.
Spread batter evenly in a greased 9 inch square baking pan, Top with the m&ms and push then into the batter a little. Not too much though, you want the pretty colors peeking through and the brownies will bake up around them.
Bake in a 325 degree F oven for about 35 minutes. Let cool in pan on a rack. Cut into 2 1/4 inch squares. Store in airtight container.