1cupmilk chocolate melting wafers(150g) or you may substitute 1 cup milk chocolate chips and 1 teaspoon shortening
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.
Microwave in 15-second increments (stirring between) until butter is completely melted.
Stir in cocoa powder until completely combined.
Add sugar and stir well.
Add eggs, vanilla extract, and water. Stir well.
In a separate bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt.
Gradually add dry ingredients to wet, stirring until completely combined.
Scoop cookie dough by 1.5 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.
In a stand mixer, beat butter until well-creamed.
Add 1 cup of sugar and cocoa powder. Sprinkle in salt. Stir until combined.
Add 2 Tablespoons of heavy cream, stir well.
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.
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.
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).