Preheat the oven to 350 F degrees. Place a silicone baking mat or parchment paper onto cookie sheets.
In a mixing bowl or the bowl of your stand mixer, cream together the butter, sugar and powdered sugar.
Add the oil and eggs and mix until well combined.
Add the cornstarch, baking soda, cream of tartar, salt, cocoa and flour. Mix to combine.
Roll balls of dough into 1 ½ inch balls and place onto cookie sheets.
In a shallow bowl place remaining ¼ cup sugar. Dip the bottom of a smooth glass into water, then into the sugar and flatten cookies until the edges begin to crack.
Bake for 10 minutes and remove from oven. Place cookies on wire racks and allow to cool. Place in the fridge until ready to frost.
Frosting
Place butter in a large bowl. Add the powdered sugar and cream and cocoa powder and beat until fluffy. Add the vanilla and beat until well combined.
Frost cold cookies and enjoy.
Notes
Pro Tips:
These cookies can be frozen! I like to freeze the cookies and the frosting separately. Then thaw frosting at room temperature or in a cool water bath when ready to frost and eat.
I always use parchment paper on my cookie trays when making cookies for best results.