Grease an 8-inch square pan with butter or generously spray with cooking spray.
Place butter and chocolate chips in a large microwave-safe bowl and microwave on high for 30 seconds. Stir and make sure all chocolate is melted. If not all the way melted, place back in microwave in 15 second increments, until chocolate is all melted when stirred.
Cool slightly and stir in sugar, salt, and vanilla. Mix in eggs one at a time. Next, add flour and cocoa powder until the mixture is smooth. Place the batter in the prepared pan and bake for 25-35 minutes, or until toothpick comes out almost clean. Set pan on rack to cool for 30 minutes.
To make frosting: Place peanut butter, butter, and chocolate chips in microwave safe bowl and microwave on high for 30 seconds. Remove and stir until all ingredients melt and combine together. Mix in powdered sugar. Gently spread evenly all over the top of your brownies. You may have extra frosting leftover. Place Peanut Butter M&Ms in horizontal lines on brownies. Cut into squares. Makes 16 brownies.