Chocolate Peppermint Sandwich Cookies

Name: Chocolate Peppermint Sandwich Cookies
Ingredients: 1 1/2 cups all-purpose flour (6 3/4 ounces; 191 g); 1/3 cup Dutch process cocoa powder (1 ounce; 28 g); 1 teaspoon Diamond Crystal kosher salt; for table salt, use half as much by volume; 3/4 teaspoon baking powder; 1/3 cup (80 ml) refined coconut oil, at room temperature (see notes); 2/3 cup granulated sugar (4 2/3 ounces; 132 g); 5 1/2 tablespoons unsalted butter (2 3/4 ounces; 78 g), softened; 1 large egg yolk; 2 tablespoons (30 ml) heavy cream; 2 teaspoons vanilla extract; 1/4 teaspoon peppermint extract; 12 tablespoons unsalted butter (6 ounces; 170 g), softened; 1 3/4 teaspoons vanilla extract; 1/4 teaspoon Diamond Crystal kosher salt; for table salt, use as much by volume; 2 3/4 cups powdered sugar (11 ounces; 312 g); 3 tablespoons (45 ml) heavy cream, divided; 3 tablespoons finely crushed soft peppermint candies (22 g), plus more for serving