Preheat the oven to 356°F (180°C) and prepare a square baking pan of about 9x9 inches (22x22cm) by spreading some butter and flour, for the blondies to be easily turn out after baked.
In a large bowl, beat together the melted butter and both brown sugar and granulated sugar for about 5 min.
Beat in the egg and vanilla extract.
Now, using a rubber spatula, mix in the flour, baking powder and salt, in a folding manner, from the outside in.
After it is all well mixed, stir in the m&m´s, pour the mixture in the baking pan and bake at 356°F (|80°C) for 20 to 25 minutes.
Once baked, and cooled, cut the blondies into 16 squares.