In a large bowl, sift together flour, cocoa, baking soda, and salt. Using a mixer fitted with the paddle attachment, beat butter and sugar for at least 3 minutes until fluffy. Next, beat in the egg and vanilla extract, mixing until combined. Add the dry ingredients and beat on medium-low speed just until combined, or mix by hand with a wooden spoon. Don’t over mix. The batter will be thick and sticky.

If you prefer thick cookies, chill the dough in the refrigerator for at least 30 minutes (and up to 2 days). This will help them to spread less while baking. Otherwise, skip to the next step.

Preheat oven to 350F/180C. Line two large baking sheets with parchment paper. Set aside.

Take cookies batter out of fridge. If it’s too cold or hard to handle, leave it on the counter for a few minutes to soften.

Drop rounded balls of dough (roughly 2 tablespoons in size) onto prepared baking sheet, leaving about 3 inches (8 cm) between each cookie. Bake for 8-10 minutes, until the cookies have puffed slightly and the tops have dried. Leave to cool on the pan for 10 minutes. Carefully transfer to a wire rack to cool completely.

To make the sandwiches: Using an ice cream scoop, take a large scoop of ice cream and sandwich it between two cookies, pressing gently. You can roll the sides in sprinkles, crushed nuts or mini chocolate chips. Serve the sandwiches immediately or place them in an airtight container and freeze. Take the sandwiches out of the freezer 5 minutes before serving.