Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a large bowl, cream together butter, granulated sugar, and brown sugar until light and fluffy. Beat in egg and vanilla extract.
In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt. Gradually add to the wet mixture, mixing until just combined.
Drop dough by rounded tablespoons onto prepared baking sheet, spacing about 2 inches apart. Flatten slightly with the back of a spoon.
Bake for 8-10 minutes, until edges are set. Cool on baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
Once cookies are cool, spread about 1/4 cup of softened ice cream onto the flat side of one cookie. Top with another cookie, flat side down, and gently press together. Repeat with remaining cookies.
Wrap each sandwich individually in plastic wrap and freeze for at least 2 hours before serving.
Tips
For best results, use a cookie scoop for uniform size.
Store sandwiches in the freezer for up to 2 weeks.
Let sandwiches sit at room temperature for 2-3 minutes before eating for easier bite.