Creating Chocolate Crinkle Cookie Sandwiches with Mint Chip Buttercream is an enjoyable process. Follow these steps for success:

1. Preheat the Oven: Set your oven to 350°F (175°C). Line two baking sheets with parchment paper.
2. Mix Dry Ingredients: In a bowl, combine flour, cocoa powder, baking powder, and salt. Set aside.
3. Cream Butter and Sugars: In a separate large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
4. Add Egg and Vanilla: Beat in the egg and vanilla extract until well combined.
5. Combine Wet and Dry Mixtures: Gradually add the dry ingredients to the wet mixture, stirring until just combined.
6. Chill the Dough: Refrigerate the cookie dough for about 30 minutes to help it firm up.
7. Form Cookies: Using a tablespoon, scoop out portions of dough and roll them into balls. Roll each ball in powdered sugar until fully coated.
8. Bake Cookies: Place the balls on the prepared baking sheets, spacing them about 2 inches apart. Bake for 12-15 minutes, or until the tops are crinkled and slightly firm.
9. Cool Cookies: Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.
10. Prepare the Buttercream: While the cookies cool, beat the softened butter in a bowl. Gradually add in powdered sugar, peppermint extract, and enough milk to achieve a creamy consistency. Fold in mini chocolate chips.
11. Assemble Sandwiches: Once the cookies are cool, spread a generous amount of mint chip buttercream on the flat side of one cookie. Top with another cookie, flat side down, to form a sandwich.
12. Serve: Enjoy immediately or store in an airtight container for later.