Mix butter and sugars until light and fluffy, a couple minutes (I used a stand mixer).
Blend peanut butter into mixture.
Add vanilla and eggs; mix until well combined.
In a separate bowl, whisk together flour, baking soda, and salt.
Add flour to peanut butter mixture and mix well.
Mix M&M's into dough by hand.
To form cookies, scoop ⅓ cup dough and roll it into a ball.
Place dough on cookie sheet and flatten (they should be somewhere between ¼ to ½-inch thick).
Bake cookies for about 12 minutes or until golden brown on bottom.
Remove cookies from oven, then let cool on pan for 5 minutes. (If desired, decorate cookies with additional M&M's as soon as you remove cookies from oven.)
Transfer cookies to a wire rack and let cool completely before proceeding.
Cover bottom side of cookie with ⅓ cup ice cream.
Top with another cookie.
Repeat with remaining cookies, then freeze ice cream sandwiches until solid, about 2 hours.
Once frozen, serve ice cream sandwiches immediately or wrap in plastic wrap or parchment paper to save for later. Enjoy!
Notes
To make smaller ice cream sandwiches, use ¼ cup dough for cookies.