Triple Chocolate Sandwich Cookies

Preheat the oven to 375°F and line a large baking sheet with parchment paper and set aside.
In a medium bowl, sift together the flour, cocoa, salt, baking powder, and baking soda and set aside.
In a large bowl or stand mixer fitted with a paddle attachment, cream together the butter and sugar until light and fluffy, about 3 to 4 minutes.
Add in the egg and vanilla extract and beat just until combined.
Add the dry ingredients to the wet ingredients and mix just until combined, scraping down the sides of the bowl as needed.
Measure the dough out into 1½ tablespoon portions and roll into balls with your hands. Place the balls about 2 inches apart on the prepared baking sheet and use your palm or the bottom of a glass to press the cookies flat. They should be about ⅓-inch thick.
Bake the cookies for 7 to 9 minutes, remove from the oven and allow to cool for 3 minutes on the baking sheet before transferring to a wire rack to cool completely.
Once the cookies have completely cooled, use a pastry bag and piping tip to add about 1 tablespoon of the chocolate frosting to half of the cookies, then sandwich the filling with the remaining cookies.
In a small bowl, melt the milk chocolate chips on 30-second intervals in the microwave, stirring after each one, until fully melted. Dip each sandwich cookie halfway in the chocolate and then placing them on wax paper to set.
Before the chocolate hardens, decorate the cookies with holiday sprinkles.