Preheat the oven to 180°C (350°F) and line an 9"x9" baking tin.
Melt the butter and mix the with brown sugar.
Add the egg and vanilla and beat in.
Stir in the flour, baking powder and baking soda until all combined.
Stir in all the chocolate chips and M&Ms and set aside.
Brownies;
Melt the butter and dark chocolate together.
Mix the eggs, sugar, brown sugar and vanilla extract together until the mixture becomes a light golden colour.
Mix in the chocolate mixture.
Stir in the flour, cocoa and M&Ms until just combined.
Spread the mixture into the prepared pan in an even layer.
Pour over the blondie layer on top and spread in an even layer.
Place in the oven to bake for 20-25 minutes. The top and sides will be set and the middle will have crumbs attached to a skewer inserted but should not be wet.