• Preheat oven to 350°F.
• In a microwave-safe bowl, melt the 1 cup unsalted butter (i.e. 2 sticks, i.e. 2 sticks) for 30-60 seconds or until fully melted.
• Add the ½ cup granulated sugar and 1 cup light brown sugar to a medium size bowl then add melted butter. Use a spatula to stir until combined (*do not use an electric mixer!).
• Add in 2 large eggs and 1 tablespoon vanilla extract, then mix. Next, add in 1 teaspoon baking soda, ½ teaspoon baking powder, and ½ teaspoon salt and mix again.
• Add in 3 cups all-purpose flour and mix until all the flour disappears into the dough.
• Line a light metal baking sheet with parchment paper. Scoop dough into 2-tablespoon-sized scoops (I use this stainless steel cookie scoop!).
• Bake at 350℉ for 9-10 minutes. The key to the perfect soft & chewy texture is to take them out of the oven before the edges turn brown. The cookies will continue to bake on the hot pan, allowing them to firm up without overbaking.
• Immediately after removing from the oven, press on the Mini m&ms. You want to work fast, as the m&m's won't stick once the cookie cools.
• Once all your m&m's are added, allow cookies to fully cool on a cooling rack before frosting.

Make Frosting

• Beat the softened ½ cup unsalted butter , i.e. 1 stick) with a hand mixer on medium-high speed or 2 minutes until smooth. Turn the handheld mixer to low speed then add the powdered sugar ¼ cup at a time, 2 ½ cups powdered sugar total. You may need to stop and scrape down the sides.
• Add 2 teaspoons vanilla extract and ⅕ teaspoons salt and turn mixer to medium speed. One tablespoon at a time, add milk or heavy cream (2-3 tablespoons total). Beat on high for 2-3 minutes until creamy and the frosting is at a good consistency to pipe. If it looks too runny, add more powdered sugar. If it looks too stiff, add a tiny bit of milk until you get to the desired consistency.

Assemble Cookie Sandwiches

• Once cookies are completely cool, pipe frosting (2 tablespoons) on the flat side of half of the cookies. Place remaining cookies, flat side down, on top of buttercream frosting and enjoy!