Step 1

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

Step 2

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

Step 3

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.

Step 4

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

Step 5

Use a wooden spoon to stir chocolate through.

Step 6

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.

Step 7

Refrigerate 1.5 - 2 hours (Note 4).

Step 8

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

Step 9

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

Step 10

Remove from fridge, unwrap.

Step 11

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.

Step 12

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

Step 13

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

Step 14

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

Step 15

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