01 -
Preheat oven to 350°F and line two baking sheets with parchment paper. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar on medium-high speed until light and fluffy, about 2 to 3 minutes, scraping down the sides as needed. Beat in the egg, corn syrup, and vanilla extract.
02 -
In a medium bowl, whisk together the flour, cocoa powder, cornstarch, baking soda, and salt. Gradually add the dry ingredients to the wet mixture on low speed until combined, forming a soft, slightly sticky dough.
03 -
Using a 1 ½-tablespoon spring-loaded scoop, drop balls of dough onto the prepared baking sheets. Bake for 8 to 9 minutes or until the cookies are slightly puffed and edges are set. Let them cool on the sheets for a few minutes before transferring to a cooling rack. Cookies will flatten as they cool.
04 -
Microwave milk chocolate chips and vegetable oil in a microwave-safe bowl, stirring every 15 seconds until smooth. Drizzle or pipe the chocolate over the cooled cookies. Refrigerate for at least 30 minutes until the chocolate sets.
05 -
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth. Add the sifted cocoa powder and powdered sugar, then beat until light and fluffy, about 3 minutes. Add the warm water and vanilla extract, and beat on medium-high speed for about 2 more minutes.
06 -
Pipe or spread the filling onto the flat side of half the cookies. Gently press another cookie on top of each one to create sandwich cookies.
07 -
Serve immediately or store in an airtight container. Keep at room temperature for up to 1 day, refrigerate for up to 5 days, or freeze for up to 1 month. Serve cold for a fudgier texture.