In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.

Beat in the egg and vanilla extract until well combined.

In a separate medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. The dough will be stiff.

Divide the dough in half, flatten each half into a disk, wrap in plastic wrap, and refrigerate for at least 30 minutes.

Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.

On a lightly floured surface, roll out one disk of dough to about 1/8-inch thickness.

Using a round cookie cutter (about 1.5-2 inches in diameter), cut out circles. Re-roll scraps as needed.

Place cookies on the prepared baking sheets, about 1 inch apart.

Bake for 10-12 minutes, or until the edges are firm. Be careful not to overbake.

Let the cookies cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.

In a medium bowl, beat the softened butter until creamy.

Gradually beat in the powdered sugar, then the vanilla extract. Add milk or cream, one tablespoon at a time, until the filling is light and fluffy.

Once the cookies are completely cool, spread a generous amount of the cream filling onto the flat side of one cookie.

Top with another cookie, pressing gently to sandwich the filling.

Repeat with the remaining cookies. Store in an airtight container.