Preheat your oven to 180°C and line a 9x9 inch baking tray.
In a bowl, combine the softened butter, caster sugar, and brown sugar. Use a whisk or wooden spoon to mix until well combined. Add the eggs and vanilla extract, and mix until smooth.
Add the flour, baking powder, baking soda and salt into the wet ingredients. Mix until all combined and a thick dough forms.
Gently fold in the Christmas M&Ms and chocolate chips, reserving a few to sprinkle on top.
Transfer the dough to the prepared baking dish and press it evenly into the pan. Sprinkle the reserved M&Ms and chocolate chips over the top.
Bake for 25–30 minutes or until the edges are golden brown. Allow the blondies to cool completely before slicing into 9 larger pieces or 12 smaller pieces. Enjoy!