Place the chocolate and butter in a heatproof bowl set over a pot of simmering water (double boiler) or in a microwave‑safe bowl. Melt, stirring often, until smooth and combined, then set aside to cool slightly.
In a separate bowl, beat the egg with the sugar until the mixture is pale, thick, and fluffy. Fold in the cooled chocolate mixture. Sift the flour, mix it with the baking powder, then gradually add this dry mixture to the chocolate batter, stirring just until combined.
The dough will be sticky. Cover the bowl with plastic wrap and refrigerate the dough for at least 15 minutes so it firms up and becomes easier to shape.
Roll the chilled dough into equal‑sized balls, place them on a baking sheet lined with parchment paper, and gently flatten the tops. Bake at 350°F (180°C) for about 15 minutes. Do not overbake: the cookies should be crisp on the outside while still soft and tender inside.
For the cream filling, first beat the softened butter with the powdered sugar until light and creamy. Then add the cream cheese and beat again until smooth. Chill the cream in the refrigerator for about 30 minutes to firm up before spreading.
Transfer the chilled cream cheese filling to a piping bag. Pipe the cream onto half of the cookies, top with the remaining cookies to form sandwiches, and enjoy.