Preheat your oven to 350°F (180°C) and line three baking trays with parchment paper.
In a bowl, sift together flour, cocoa powder, salt, and baking soda. Set aside.
Cream butter and sugars with a mixer until light and fluffy (3–4 minutes).
Add egg and vanilla to the butter mixture, beating until combined.
Mix in the dry ingredients on low until just combined.
Scoop 2 tablespoons of dough (40g), roll into balls, coat in sugar, and place on trays with space to spread.
Bake one tray at a time for 11–12 minutes until puffed and crackly. Cool on the tray for 15 minutes, then transfer to a rack.
Store in an airtight container at room temperature for up to 5 days.