Place the dark chocolate and butter in a heatproof bowl set over a pot of simmering water or use the microwave, and melt them together, stirring constantly until smooth and combined. Let the mixture cool slightly.

Beat the egg with the sugar until the mixture becomes pale and fluffy. Fold in the cooled chocolate mixture. Sift in the flour and mix it with the baking powder, then gradually add this dry mixture to the chocolate mixture.

The dough will be sticky, so cover it with plastic wrap and refrigerate it for at least 15 minutes.

Roll the chilled dough into balls of equal size, place them on a baking sheet lined with parchment paper, gently flatten the tops, and bake. Bake for about 15 minutes at 350°F (180°C). The main thing is not to overbake them: they should be crisp on the outside but still soft and tender inside.

For the cream, first beat the room‑temperature butter with the powdered sugar until smooth, then add the cream cheese and beat again until creamy. Chill the cream in the fridge for about 30 minutes before spreading.

Transfer the cream to a piping bag, pipe it onto half of the cookies, sandwich them with the remaining cookies, and enjoy.