Beat softened butter and sugars with an electric hand mixer or stand mixer until smooth.
Next add in your egg and vanilla extract then mix again on low until combined.
In a separate bowl combine your flour, baking soda, and salt. Once combined gradually add this to your wet ingredients. Add in about ¾ of a cup of the dry ingredients at a time to your wet and mix to combine before adding in more.
Mix until just combined then add in ¾ cup of M&Ms and fold into batter.
This batter could be thick and sticky. Place in the fridge to chill for at least 2-3 hours.
Preheat the oven to 350 degrees F. Prepare a baking sheet with nonstick cooking spray and set aside.
Once chilled scoop out 2 tablespoons of batter and roll into 2-inch balls.
Place cookie dough balls on your prepared baking sheet about 2 inches apart. Gently press in remaining M&Ms to the top of each cookie. Carefully not to press down the cookie.
Bake for 10-12 minutes.
Remove from the oven and allow to stand on the pan for 5-10 minutes before serving.
Tip: Flattening the cookies will cause them to spread out more during baking, making them crispier.