In a mixer or medium bowl, beat the sugar and butter together, and then add the eggs and vanilla extract. Stir until combined.

In a separate medium bowl, whisk the flour, baking powder, and salt and then gradually beat into the butter mixture until incorporated.

Remove half of the dough from the mixing bowl and set aside.

Add the cocoa powder to the remaining dough and beat until evenly mixed.

On a large piece of parchment paper, roll the plain dough into a 9x14-inch (23x36cm) rectangle.

On a separate piece of parchment, roll the chocolate dough into a 9x14-inch (23x36cm) rectangle.

The doughs are very soft, so use the parchment paper to help you lift the chocolate dough and place it on top of the plain dough – so now you should have both doughs together with a piece of parchment below the plain dough and on top of the chocolate dough.

Gently roll the doughs together to a thickness of about ¼-inch (6mm).

Peel away the top sheet of parchment paper and discard.

Starting on a long side, carefully roll the dough into a tight roll and then use the bottom parchment paper to wrap the dough and refrigerate until very firm, at least 1 hour or overnight.

When ready to bake, preheat the oven to 350°F (180°C) and line two baking sheets with parchment paper.

Unwrap the cookie log and with a sharp knife, slice ¼-inch (6mm) rounds and place on the prepared baking sheets about 1-inch (2½cm) apart.

Bake for 8-10 minutes, until just cooked through. They should pale and not take on any color.

Let cool on wire racks and then store in an airtight container at room temperature for up to 3 days.