Preheat your oven to 350°F (180°C) and line baking sheets with parchment paper.

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

In a larger bowl, cream together softened butter and sugar until light and fluffy.

Add the egg and vanilla, and beat until well combined.

Pour in the cooled coffee, and gradually mix in the dry ingredients until everything is well incorporated.

Drop small mounds of dough onto the prepared baking sheets, leaving space between each.

Bake for 10-12 minutes, or until the edges look set but the centers remain soft.

While the cookies cool, combine the softened butter and powdered sugar in a mixing bowl and beat until creamy.

Add in the vanilla and brewed coffee, mixing until smooth and spreadable.

Once cookies are cool, flip half of them upside down and spread or pipe a generous spoonful of filling on each.

Top with the other half, pressing down gently to sandwich the filling.