Place butter, salt and both sugars in a bowl. Beat on speed 5 for 1 minute until it's soft, creamy and fluffy.

Add yolks and vanilla - beat for 1 minute until well incorporated.

Add baking powder, rice flour and half the plain flour. Beat until you can't see flour anymore, then add remaining flour and beat again until incorporated.

Mixture will be fairly clump and thick, but if you press between your fingers, it should stick together (rather than being dry and crumbly).

Use a wooden spoon to stir chocolate through.

Tip out onto a work surface then press together into a 22cm / 9" log. Wrap in cling wrap or paper, twisting to seal the ends.

Refrigerate 1.5 - 2 hours (Note 4).

Preheat oven to 200°C/390°F (180°C fan).

Line 2 trays with baking/parchment paper. Put one shelf in the middle of the oven, and the other underneath.

Remove from fridge, unwrap.

Use a serrated knife to slice into 1.75cm / 2/3" thick slices. Saw carefully through choc chunks. If it falls apart on edges, just press if back together, no big deal. Place 6 on each tray.

Bake 10 minutes. Turn oven down to 170°C/340°F (150°C fan), switch trays (Note 5)

Bake for a further 15 minutes until surface is light golden and edges are a bit golden.

Remove from oven and cool completely on trays - this makes them crunchy.

STORAGE: Keeps for at least 2 weeks in an airtight container - they stay 100% crunchy, just like they're freshly made.