Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a mixing bowl, cream together 10 tablespoons of softened unsalted butter and 3/4 cup of granulated sugar until light and fluffy. Add in one large egg, 1 tablespoon of light corn syrup, and 1 teaspoon of vanilla extract, mixing until well combined. Gradually incorporate 1 1/4 cups of all-purpose flour, 6 tablespoons of cocoa powder, 1 teaspoon of cornstarch, 1/2 teaspoon of baking soda, and 1/4 teaspoon of fine sea salt. Fold in 1/2 cup of milk chocolate chips.

Using a 1 ½-tablespoon scoop, drop balls of dough onto the prepared baking sheets, spacing them about 2 inches apart. Bake for 8-9 minutes until slightly puffed and the edges are set.

Cool cookies on the baking sheets for 5 minutes before transferring them to wire racks to cool completely.

To make the filling, beat 1/4 cup of softened unsalted butter until smooth. Gradually incorporate 1 1/2 cups of sifted powdered sugar and 2 tablespoons of sifted cocoa powder. Slowly mix in 1-2 tablespoons of warm water and 1 teaspoon of vanilla extract.

Spread a layer of filling on the flat side of one cooled cookie, top with another cookie, and gently press together. Repeat with all cookies.