• Preheat oven to 350F (177C) and line a cookie sheet with parchment paper.  Set aside.
• Cut butter into Tablespoon-sized pieces and place in a large microwave-safe bowl.  
  10 Tablespoons unsalted butter
• Microwave in 15-second increments (stirring between) until butter is completely melted.
• Stir in cocoa powder until completely combined. Make sure the butter/cocoa is no longer warm to the touch before proceeding with the recipe.
  ⅔ cup natural cocoa powder
• Add sugar and stir well.
  1 ⅔ cup sugar
• Add eggs and vanilla extract and stir well.
  2 eggs + 1 egg yolk, 1 teaspoon vanilla extract
• In a separate bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt.
  1 ⅓ cup all-purpose four, 2 teaspoons cornstarch, ½ teaspoon baking powder, ½ teaspoon baking soda, ¾ teaspoon salt
• Gradually add dry ingredients to wet, stirring until completely combined.
• Scoop cookie dough by 1 ½ Tbsp-sized scoops and drop by rounded spoonful onto prepared cookie sheet, placing cookie dough scoops at least 2" apart.
• Bake on 350F for 8-10 minutes.  Allow cookies to cool completely on cookie sheet before filling.

Frosting

• In a stand mixer, beat butter until well-creamed.
  ½ cup salted butter
• Gradually stir in about half of the powdered sugar and the cocoa powder. Stir until combined.
  2 cups powdered sugar, 3 Tablespoons natural cocoa powder
• Drizzle in about half of the heavy cream, stir well.
  ¼ cup heavy cream
• Gradually stir in remaining powdered sugar, cream, and vanilla extract.
  ½ teaspoon vanilla extract
• Pair off your cooled cookies into sandwich pairs, and then generously spread or pipe about 1 ½ Tablespoons of frosting onto the bottom of one cookie (I just use a clean 1 ½ Tablespoon cookie scoop), sandwiching with the bottom of another, until all cookie pairs are filled.
• Combine chocolate chips and shortening and melt in 15-second increments, stirring well in-between, until chocolate is completely melted.  
  ½ cup milk chocolate chips, ½ teaspoon shortening or coconut oil
• Pour chocolate into small Ziploc bag and snip a small piece out of the corner.  
• Pipe melted chocolate over cookies.  Allow chocolate to harden before serving (melting wafers chocolate will harden much faster than chocolate chip/shortening chocolate).