Preheat your oven to 350°F (175°C).
In a bowl, cream together the softened butter, granulated sugar, and brown sugar until smooth.
Add the egg and vanilla extract, mixing until well combined.
In a separate bowl, whisk together the flour, cocoa (if using), and salt.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Fold in chocolate chips or nuts if desired.
Drop spoonfuls of dough onto a lined baking sheet, spacing them about 2 inches apart.
Bake for 10-12 minutes, until edges are set but centers are soft.
Remove from the oven and let cool on the baking sheet for 5 minutes.
Once cooled, scoop your favorite ice cream onto one cookie, then top with another cookie to form a sandwich.