In the bowl of a stand mixer, blend sugars and butter to form a grainy paste.
Add the egg and vanilla. Beat at medium speed until light and fluffy.
Add flour, salt, baking soda, crisped rice cereal and chocolate. Blend on low-speed just until combined.
Roll into 1 1/2 inch balls and place in mini muffin tins that have been sprayed with cooking spray.
Bake at 350 degrees for 11 minutes.
As soon as the cookies come out of the oven, use the rounded tip of a kitchen utensil to make a well in the center of the cookie.
Allow the cookies to cool in the pan for 5 minutes, then remove them to wire racks to cool completely.
While the cookies are cooling, prepare the buttercream. Using a mixer, whip the butter for 1-2 minutes, until it is nearly white. Add in the vanilla, powdered sugar and milk. Beat the frosting on medium speed for 2 minutes until it is fluffy.
Pipe a dollop of frosting into each cooled cookie. Top with 3 M&M'sÂ® Crispy.
Store in an airtight container.
*I used Wilton tip 1M and a decorating bag to fill the cookie cups. You can use just a decorating bag, a Ziploc bag with one of the corners cut off, or just use a small spoon to fill them. *As a time-saver, use your favorite store-bought frosting.