Preheat the oven: preheat the oven to 180°C / 350-375°F / Gas Mark 4-5.
Baking dish: lightly grease a square (24 x 24 cm or 9.4 inch) baking dish with cooking oil then line it with 2 overlapping sheets of baking paper with a 2-3 inch overhang on each side.
Sugar and butter: add each of the brown sugar and white sugar to a large mixing bowl. Then transfer the butter to a heatproof jug and heat in the microwave using the high setting for 30 second increments till melted (around 1 ½ minutes). Once the butter is melted, use an oven mitt around the hand of the jug and remove it from the microwave. Set it on a heatproof surface and use a spatula to give it a gentle stir. Let it cool for a minute or so (use this time to crack the eggs into a small bowl). Then pour the melted butter into the bowl and stir until consistently combined.
Vanilla and eggs: next add the eggs and vanilla to the bowl and stir till consistently combined.
Flour: then add the flour to the bowl and stir till it is mixed in with no visible flour at the bottom of the bowl.
Mix ins: next pour the m and m's and chocolate chips into the blondie batter and gently stir to fold these in.
Transfer: transfer the combined blondie batter into the prepared baking dish. Then use a spatula to push it so that it is in a reasonably even layer and fills all 4 corners of the dish.
Bake: place the blondies into the oven (middle shelf). Bake them for 35 minutes. Once the cook time is up, use oven mitts to remove them from the oven and set them onto a heatproof surface.
Cool: rest the blondies in the baking dish still for 20 minutes. After this time, grab the overhanging parchment paper and lift the slab up and out of the baking dish. Place this onto a cooling track and let them cool for a further 20 minutes.
Portion: transfer the blondies slab to a clean, dry cutting board. Use a sharp knife and cut this into 16 equal pieces.