Place the sugars, softened butter and vanilla extract in a large bowl (or the bowl of your stand mixer fitted with the paddle attachment), and cream together until the sugar is dissolved into the butter and the mixture is light and fluffy.
Scrape down the sides of the bowl and add the egg, blend together until combined.
In a separate bowl whisk together the flour, baking soda and salt. Add the dry ingredients ½ cup at a time to the wet ingredients until combined, scraping down the sides of the bowl as needed.Add in the plain Easter M&M candies and mix until incorporated.
Scoop the cookie dough into rounded golf-ball size balls and place on a cookie sheet. Cover with plastic wrap and place in the fridge for 2 to 24 hours prior to baking for best results.
PREHEAT OVEN TO 375 F
After chilling the dough, place the cookie dough balls on a silicone mat or parchment lined baking sheet, at least 2 inches apart.
Bake cookies for 10 to 12 minutes, or until the bottom edges are lightly golden brown (the cookie may not look completely cooked yet and that's ok!). Remove from the oven and let sit on the cookie sheet for an additional 2 to 3 minutes before transferring to a wire rack to cool completely.
Notes
Store baked cookies in an air tight container on the counter for 3 to 7 days, in the fridge for 1 to 2 weeks, or in the freezer for up to 3 months. Store unbaked cookie dough in the fridge for up to 7 days or freeze for up to 3 months.