- Line baking sheets with parchment paper. Set aside. 
- In the bowl of a stand mixer with a paddle attachment (or using a handheld mixer), beat the butter on medium-high speed until smooth (about 1 minute). 
- Add peanut butter. Beat for about 2 minutes, or until smooth. 
- Add brown sugar. Cream together on medium-high speed for 2 minutes, until smooth. 
- Add eggs, one at a time, mixing for 30 seconds after each addition. Scrape down the sides of the bowl and mix again if needed. 
- With the mixer off, add flour, oats, baking soda, and salt. Mix on low until just combined, being careful not to overmix. 
- With the mixer still on low (or by hand), add the M&M's, chopped peanut butter eggs, and chocolate chips. Mix until incorporated. (If using an electric mixer, only mix for a few seconds.) 
- Using a 2-tablespoon scoop, drop dough onto the lined baking sheets about 2 inches apart. Top with extra Easter M&M's (optional). 
- Chill the dough in the refrigerator for 30 minutes. (If chilling overnight, cover the dough.) 
- When ready, preheat the oven to 350°F. Remove dough from the refrigerator. 
- Bake for 10 minutes, or until the edges are lightly browned and the centers still look soft. 
- Let the cookies cool on the baking sheet for a few minutes, then transfer to a wire rack to finish cooling.