Preheat the oven to 350°F. Line a baking sheet with parchment paper.

In a heatproof bowl, melt the chocolate and butter together until smooth, either over a pan of simmering water or in the microwave in 30-second bursts.

Sift the cocoa powder into the melted chocolate mixture and stir until combined. Let it cool slightly.

In a separate bowl, whisk the egg with both sugars until light and fluffy, about 4 minutes.

Fold the chocolate mixture into the egg mixture until just combined.

Gently mix in the flour and salt until the batter is smooth.

Spoon or pipe the batter onto the prepared baking sheet, forming 8 cookies.

Bake for 10 minutes, until the cookies are set with a crinkly top. Do not overbake.

Allow the cookies to cool on the baking sheet before serving or storing.