Preheat the oven to 350 degrees F. Line two baking sheets with parchment paper and set aside.

In the bowl of a stand mixer (or in a large bowl with an electric hand mixer), beat the butter and sugar on medium speed for 3 minutes until it is light colored and fluffy.

Scrape the sides of the bowl and add egg, corn syrup, and vanilla. Mix on medium until incorporated.

Add flour, cocoa powder, corn starch, baking soda, and salt. Slowly mix increasing mixing speed from low to medium until well combined. Dough will be soft and sticky.

Use a 1 ½ tablespoon cookie scoop and scoop dough on to baking sheets 2 inches apart. I bake 12 per baking sheet.

Bake 9 minutes just until edges are set. They will flatten as they cool.

To make the filling, use an electric hand mixer to beat the butter on medium speed until it is silky smooth. Add in the cocoa powder and powdered sugar. Mix until light colored and fluffy.

Scrape down the sides and bottom of the bowl and add in the vanilla extract and water. Continue to mix on medium speed for 2 minutes.

Spread or pipe filling onto the flat side of one cookie. Top with another cookie. Repeat until all cookies have been sandwiched with filling.