Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a medium bowl, whisk together the cocoa powder, flour, baking soda, and salt.
In a large bowl, beat the butter and sugar together until light and fluffy. Add the egg and vanilla extract, mixing until combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Scoop tablespoon-sized amounts of dough onto the prepared baking sheet, spacing them about 2 inches apart. Flatten slightly.
Bake for 8-10 minutes, or until the cookies are set. Allow to cool completely.
Once the cookies are cool, place a scoop of softened black raspberry ice cream on the flat side of half of the cookies. Top with the remaining cookies, pressing down gently to form sandwiches.
Wrap each sandwich in plastic wrap and freeze for at least 1 hour before serving.