To make the cookies:
Preheat oven to 350°F. Line a sheet pan with parchment paper.
In the bowl of stand mixer fitted with a paddle attachment, or in a large bowl with a hand-held mixer, beat the butter and sugar together on high speed until pale and creamy.
Add the eggs one at a time, mixing after each, followed by the vanilla.
In a separate bowl, combine the flour, cocoa powder, baking soda and salt. Add to the butter mixture all at once and mix until incorporated.
Shape into rounded tablespoons and place on prepared sheet pan. Bake until surface is cracked and edges are dry, about 10 minutes.
Repeat with remaining cookie dough. Let cookies cool completely.
To make the frosting:
Beat the powdered sugar, butter, salt, heavy cream, and peppermint extract on low speed until it comes together.
Increase speed to high and beat for 2-3 minutes until light and creamy.
Use a clean paintbrush to paint the red gel food coloring up the sides of a large piping bag fitted with a plain or star tip, making 4-6 lines total. Fill bag with frosting.
To assemble cookie sandwiches:
Calories: 432kcal, Carbohydrates: 61g, Protein: 4g, Fat: 21g, Saturated Fat: 13g, Cholesterol: 75mg, Sodium: 164mg, Potassium: 92mg, Fiber: 2g, Sugar: 44g, Vitamin A: 643IU, Calcium: 18mg, Iron: 2mg