In a large bowl or the bowl of a stand mixer fitted with the paddle attachment, cream together the softened butter, light brown sugar, and caster sugar on medium speed for 2-3 minutes, until the mixture is light and fluffy.

Add the crunchy peanut butter and beat for another minute until fully incorporated. Scrape down the sides of the bowl.

Beat in the egg and vanilla extract until just combined. Be careful not to overmix at this stage.

In a separate medium bowl, whisk together the plain flour, bicarbonate of soda, salt, and rolled oats.

With the mixer on low speed, gradually add the dry ingredients to the wet ingredients, mixing until just a few streaks of flour remain.

Gently fold in the chocolate chips and about two-thirds of the M&M's using a spatula. The dough will be quite thick and sticky.

Cover the bowl with cling film and refrigerate the dough for at least 30 minutes, or up to an hour. I find this is the most important step for getting that perfect chewy result.

Preheat your oven to 180°C (160°C fan) and line two large baking sheets with parchment paper.

Use a medium ice cream scoop or two spoons to portion out dough balls, about 2 tablespoons each. Roll them briefly between your palms and place them on the prepared baking sheets, leaving about 5cm of space between each one. Gently press the remaining M&M's onto the tops of the dough balls.

Bake for 10-12 minutes, or until the edges are a light golden brown and the centres look slightly underdone. They will firm up as they cool.

Let the cookies cool on the baking sheets for 5 minutes before carefully transferring them to a wire rack to cool completely.