Preheat oven to 350º F (180º C). Line a large baking sheet with parchment paper.
Combine flour, oats, baking powder, baking soda, and salt in a bowl. Set aside.
Using an electric hand mixer or a stand mixer, beat the butter for about 30 seconds. Add the sugars and beat for another 30-40 seconds until creamy. Then beat in the nut butter until incorporated.
Reduce the speed to low and add the flour mixture; mix until it resembles coarse crumbs. Add the milk and vanilla, and mix until the dough comes together into a big ball.
Gently fold the M&M’s and other add-ins (except for the candy eyes) into the cookie dough.
Use a small cookie scoop (1.5 tablespoons) to measure out the dough. Roll each into a ball and place about 2 inches apart on the cookie sheet. Gently flatten the balls lightly with your hand.
Bake for 10-12 minutes, until barely golden brown around the edges but still soft in the middle.
If you are adding candy eyes, do so now, right after baking.
Cool on baking sheets for 2 minutes; remove to wire racks to cool completely.