In a small bowl, combine the flour, ⅓ C cocoa powder, baking soda and salt. Stir until well combined.
Combine 8 tablespoon butter and sugar in the bowl of an electric mixer. Beat on medium speed for 2 minutes. Scrape down the bowl.
Add the egg and vanilla. Beat about 60 seconds more. Scrape down the bowl.
Turn your mixer to the lowest setting, add in the flour mix stirring until the dry ingredients are incorporated.
Set aside the batter while you preheat the oven. You do not need to refrigerate.
Preheat your oven to 350 degrees F. Line two baking sheets with parchment paper.
Once your oven is preheated, scoop the cookie dough into balls about the size of a golf ball. Place on the sheet leaving 2-3 inches in between. They will spread a good bit.
Bake in preheated oven for 7-8 minutes until the edges are just set if you feel with your fingertip. If your cookies spread too much, you can refrigerate the dough for about 20 minutes before baking the next batch (this could be an issue if your kitchen is especially warm)
Cool the cookies on the baking sheet for 5 minutes and then remove to a wire rack to cool completely.
For the chocolate cream
Place 6 tablespoon butter, powdered sugar, 2 tablespoon cocoa powder and ½ teaspoon vanilla in the bowl of an electric mixer. Mix on a low speed until the ingredients come together and are no longer powder. Scrape down the bowl and the beater. Then return to mixing at med-high speed for 5 minutes. You will notice the mixture turning a lighter color and becoming very fluffy.
Scoop about 3 tablespoon of cream onto 9 cookie halves. You should have exactly the right amount of cream. Top with a second cookie half and gently press together to make a sandwich.
For the chocolate stripes
Place your chocolate chips in a microwave safe bowl. Heat on 50% power for 2 minutes, then stir. If the chips aren't melted, continue to cook in 30 second intervals stirring each time.
Place your cookie sandwiches on a lined baking sheet.
Use a spoon to scoop and drizzle the melted chocolate over the sandwiches, creating stripes.
Place the tray of cookie sandwiches in the refrigerator for 1 hour to set the cream center and the chocolate stripes.