In the mixing bowl beat the butter and powdered sugar until light and fluffy, approximately 3 minutes on medium high speed.
Add the cornstarch, flour, cocoa, salt and vanilla, beat for 2-3 minutes on medium high speed until well combined. Add the chocolate chips or nuts if using, beat on low speed or fold in to combine.
Drop by heaping teaspoonfuls on 1-2 parchment paper lined cookie sheets, 1 -2 inches apart, sprinkle with some sprinkles or more nuts or chips. While the oven is pre-heating refrigerate the cookies.
Pre-heat the oven to 325F (160C).
Bake the cookies one tray at a time on the middle rack for 13-15 minutes. The cookies will be soft to the touch but will firm up once cooled. Enjoy!
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Notes
You can sift the icing sugar, cornstarch and flour directly into the mixing bowl.Your baked cookies should be left to cool then store them in an airtight container at room temperature they will last up to 5 days. I like cold cookies so I actually store mine in the fridge, they will last up to 10 days in the fridge. You can also bring them to room temperature before serving.Freeze the completely cooled cookies in an airtight freezer safe container or bag. They will keep for up to 2 months in the freezer. If you are worried about the cookies sticking together, then place the cooled cookies on a cookie sheet and freeze until firm (about 2 hours) then move to the containers or bags. You can also place parchment paper between the layers.You can also freeze the dough, make the dough balls and place them on a parchment paper lined cookie sheet, once they are frozen place in an airtight freezer safe container or bag. The cookies can be baked frozen, just be sure to add an additional couple of minutes to the baking time.