Coat 2 large baking sheets with cooking spray or parchment paper, set aside.
Combine butter, sugars and vanilla in the bowl of an electric mixer and beat on medium speed until light, about 5 minutes, scraping bowl and beaters occasionally.
Beat in the egg until smooth.
While the mixture is beating, sift the cocoa with the flour and salt.
Stop mixer, scrape down bowl and beaters and add dry ingredients.
Mix on low speed until incorporated.
Scrape the dough out onto a piece of plastic wrap.
Cover with another piece of wrap and press the dough into a rough disk.
Refrigerate the dough until it is firm -- several hours or overnight.
Preheat the oven to 350 degrees, arrange rack at the middle level of the oven .
Divide the dough into 3 parts and roll one at a time on a lightly floured work surface into a 6-inch square.
Cut the dough into Halloween shapes with Halloween cookie cutters and set them on the prepared pan.
Continue with the remaining dough.
After rolling all the dough, press the scraps back together and make about 10 more cookies.
Pierce the cookies several times with a fork and sprinkle them lightly with icing sugar.
Bake the cookies about 20 minutes, until slightly puffed and firm. Cool the cookies on a wire rack, make about 3 dozens cookies.
Storage suggestion:
Keep the cookies between layers of waxed paper in an air-tight tin or plastic container.
For oreo cookie filling:
Combine filling ingredients and mix well.
Form into balls about 1/2 to 3/4 inch in diameter, again using your hands.
Sandwich one filling in the center of two cookies and carefully press down until the filling spreads almost to the edge.
