In a medium bowl, stir together the melted butter and granulated sugar and brown sugar until smooth.

Then whisk  in the egg and vanilla extract, the mixture will lighten slightly. 

Stir in the cocoa powder until smooth, then add the flour, salt and baking powder.  

Fold in the M&Ms and  the chocolate chips. 

Cover the bowl and refrigerate for at least one hour. 

Heat the oven to 325ºF. 

Spray a mini cupcake tin with cooking spray. 

Fill each mini muffin well three quarters of the way full. 

Bake for 9-12 minutes.  The brownie cups will rise slightly and the edges should look done.  Remove from the oven and set the pan on a wire rack. 

Let the brownies cool slightly in the pan, about 15 minutes before removing them from the pan.  Serve warm or at room temperature.