Texas Sheet Cake Cookies

 Preheat oven to 350º F. In an electric mixer, cream together butter and sugar until well combined and creamy.
 Add in the egg and vanilla extract and beat until everything is fully incorporated.
 Mix in cocoa powder, baking powder, salt, cinnamon, and flour until it forms a batter. Pour in the melted chocolate and stir until well combined.
 Scoop 18 balls of dough and place onto a lined baking sheet. Bake for 8 minutes, until the cookies are just dry on the top. Be careful not to over bake.
 Once the cookies are completely cool, prepare the icing. In a saucepan, melt the butter. Add cocoa powder, milk, vinegar, and powdered sugar, whisking until smooth and pourable. If the mixture is too thick, add more milk, 1 teaspoon at a time, to get to a pourable consistency.
 Pour the frosting over the cooled cookies on the baking tray. Let the cookies sit for a couple of hours for the icing to completely set.

Notes :

 The cinnamon in the recipe is optional but adds a classic Texas Sheet Cake flavor.
 Do not over bake the cookies to keep them fudgy.
 Store cookies in an airtight container at room temperature for up to 3 days.