Add butter and sugars to a large bowl and mix on medium speed with a hand mixer until light and fluffy. Add egg, vanilla extract, and salt. Beat until well incorporated.
Combine flour, cocoa powder and baking soda in a separate bowl. Whisk together until well combined.
Switch the hand mixer to a low speed and slowly add the dry ingredients to the wet ingredients. The batter will be thick – mix only until everything is combined.
Spoon the cookie dough onto your baking sheets and bake for 9 to 12 minutes. Cool on baking sheets for 5 minutes, then transfer to a cooling rack.
While the cookies are cooling, line a baking sheet with parchment paper and scoop ice cream into balls – as many sandwiches as you want to make. Once you have all your scoops, immediately put the baking sheet in the freezer.
When the cookies have completely cooled, remove the ice cream from the freezer. Using a piece of parchment paper and your palm, gently push down on each ice cream ball to flatten it slightly and fit the width of your cookies. Place the pressed ice cream balls between two cookies, firmly but gently enough not to break the cookies.
Serve and enjoy! Or you can wrap them in parchment paper, plastic or foil to preserve them in the freezer until needed.