Preheat oven to 350 degrees F (175 degrees C). Grease and flour an 8x8 inch pan.
In a large bowl, cream together the butter and sugar until smooth.
Beat in the egg and vanilla.
Combine the flour, cocoa, baking soda and salt; gradually stir into the creamed mixture.
Stir in the chocolate chips.
Spread evenly into the prepared pan.
Bake for 25 to 30 minutes in the preheated oven. Allow to cool before serving.
To assemble ice cream sandwiches, scoop softened ice cream onto cooled cookie. Place another cookie on top of ice cream. Gently press together. Wrap each sandwich in plastic wrap or parchment paper and freeze until firm before serving.