For the cookies:
- 1 cup plus 1 tbsp. (5¼ oz.) all-purpose flour
- ¼ tsp. baking soda
- ¼ tsp. salt
- 6 tbsp. (3 oz.) unsalted butter, melted and cooled until warm
- ½ cup (3½ oz.) packed brown sugar
- ¼ cup (1¾ oz.) granulated sugar
- ½ large egg plus ½ egg yolk (or 1 medium egg)
- 1 tsp. vanilla extract
- ½ cups mini M&Ms, plus extra for topping
For the filling:
- 6½ tbsp. unsalted butter, at room temperature
- ¾ cup confectioners’ sugar, sifted
- Pinch of salt
- 1½ tbsp. heavy cream
- ¾ tsp. vanilla extract
step 1: Show Images
Adjust oven racks to upper and lower-middle positions. Preheat oven 325˚ F. Line two cookie sheets with parchment paper. Whisk dry ingredients together in a medium bowl; set aside. With electric mixer, or by hand, mix butter and sugars until thoroughly combined. Beat in egg, yolk, and vanilla until combined. Add dry ingredients and beat at low-speed just until combined. Fold in the M&Ms with a spatula.
step 2: Show Images
Roll a tablespoon of dough into a ball. Holding dough ball in fingertips of both hands, pull apart into two equal halves. With the torn surface facing up, place formed dough onto cookie sheet. Top each portion with a few extra mini M&Ms. Bake, rotating the baking sheets halfway through baking, until cookies are light golden brown and outer edges start to harden yet centers are still soft and puffy (approximately 10-12 minutes). Do not overbake.
step 3: Show Images
To make the filling, place the butter in the bowl of an electric mixer fitted with the whisk attachment. Whip on medium-high speed until smooth, about 1 minute. Add the confectioners’ sugar to the bowl and mix on medium-low speed just until incorporated. Add in the salt. Continue to beat on medium-high speed until smooth, about 1-2 minutes. Mix in the heavy cream and vanilla on low speed just until incorporated. Increase the mixer speed and whip on high speed until light and fluffy, scraping down the bowl as needed, about 4-5 minutes.
step 4: Show Images
Pair up the cooled cookies by size. Transfer the filling to a pastry bag fitted with a plain round tip to pipe a dollop of filling onto the flat side of one cookie of each pair. Sandwich together with the remaining cookie and press down so that the filling reaches the edges. Store in an airtight container.