In a large bowl, beat the butter, peanut butter, brown sugar and granulated sugar with a mixer on medium speed for 2 minutes, until creamy and slightly lighter in color. Use regular creamy peanut butter, since natural style can make the cookies oily and crumbly.
Beat in the milk and vanilla until smooth. The mixture may look a little loose at this point, which is completely normal.
Add the flour, baking soda and salt and mix on low just until combined. Stir in the oats, chocolate chips and 1/2 cup of the M&M candies with a spatula.
Cover the bowl and chill the dough for 20 minutes. This gives the oats time to soak up moisture so the cookies hold together beautifully without eggs.Near the end of chilling, preheat the oven to 350°F and line two baking sheets with parchment paper.
Scoop the dough into 2 tbsp balls, place them 2 inches apart and flatten each one slightly with your palm. Press the remaining 1/4 cup M&M candies onto the tops.
Bake for 9-11 minutes, until the edges are lightly golden and the centers still look soft. Avoid overbaking, as they firm up while cooling.
Cool on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely. Makes about 18 cookies.