Preheat the oven to 350°F (175°C). Line a mini muffin pan with 18 mini paper cups. Set aside.
In a medium mixing bowl, combine gluten-free flour blend, baking soda and salt. Whisk to combine. Set aside.
In a large mixing bowl, add room temperature butter, brown sugar, and granulated sugar.
Cream with an electric mixer for 2-3 minutes, until light and fluffy, scraping down the sides of the bowl with a spatula as needed.
Add the egg and vanilla extract and mix until smooth.
Add half of the flour mixture to the butter mixture and stir until just combined. Repeat with the remaining flour, stirring until no more streaks of flour remain.
Stir in the mini chocolate chips.
Spoon the dough into each muffin well.
Bake at 350F for 10-12 minutes, until the cookies are puffed up and lightly browned. Do not overbake.
Remove from the oven and immediately press the mini peanut butter cups down into the cookies.
Once the chocolate on the mini peanut butter cups has melted from the heat of the cookies, add 3 M&Ms to the top of each cookie. Let the cookies cool completely before removing from the muffin wells.