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 (142 g) 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 (60 g) natural cocoa powder

Add sugar and stir well.1 ⅔ cup (365 g) sugar

Add eggs, vanilla extract, and water.  Stir well.2 eggs + 1 egg yolk, 1 ½ Tablespoon water, 1 teaspoon (5 ml) vanilla extract

In a separate bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt.1 ⅓ cup (170 g) all-purpose four, 2 teaspoons cornstarch, ½ teaspoon baking powder, ½ teaspoon baking soda, ½ 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 10-12 minutes.  Allow cookies to cool completely on cookie sheet before filling.

FrostingIn a stand mixer, beat butter until well-creamed.¾ cup (170 g) butter

Add 1 cup of sugar and cocoa powder.  Sprinkle in salt.  Stir until combined.3 cups (375 g) powdered sugar, 4 ½ Tablespoons (25 g) natural cocoa powder, ¼ teaspoon salt

Add 2 Tablespoons of heavy cream, stir well.6 Tablespoons (88 ml) heavy cream

Add another cup of sugar and stir, followed by 2 more Tablespoons of cream.  Repeat with remaining sugar and cream, and then stir in vanilla extract.½ teaspoon vanilla extract

Pair off your cooled cookies into sandwich pairs, and then generously spread or pipe frosting/filling onto the bottom of one cookie, sandwiching with the bottom of another, until all cookie pairs are filled.

Melt chocolate melting wafers according to package instructions (or, if using milk chocolate chips and shortening, microwave chips and shortening at 15-second increments, stirring well in-between), until chocolate is completely melted.  1 cup (150 g) milk chocolate melting wafers

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).