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1
Whisk together the flour and baking ammonia in a large bowl then whisk in the sugar. Add the butter in small lumps, half the beaten egg, the sour cream and vanilla extract. Rub the mixture together with your fingertips until large coarse crumbs form. Keep rubbing until they start to lump together into a ball, becoming more damp than floury, gathering the remainder as you go. It will be sticky and streaky; this is fine. Wrap this lump with cling film and leave aside for at least 10 minutes, but up to 30 minutes will be okay.
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2
Arrange the racks in the oven to accommodate two trays (if necessary, otherwise just keep one rack in the middle) and preheat the oven to 200u00b0C (180u00b0C fan-forced/ 400u00b0F). Grease and line two large baking trays.
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3
Unwrap and shape the dough into a rounded, but flat log, as you would with biscotti. Not too wide. Roll it out with your hands if you find it easier. Cut even 'slices' of dough, just under a centimetre thick, and place each one on a baking tray leaving a 2-3cm gap between them.
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4
Tip the sugar into a shallow bowl. Brush the tops and sides of the biscuits with the remaining egg. Take a biscuit and press into the sugar, coating the top and side. Re-arrange back on the tray and continue for the remaining biscuits.
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5
When the oven is hot enough (mine has a light that switches off at the indicated temperature), insert the trays and bake the biscuits for 10 minutes but check after 8 minutes, or until very lightly golden in colour. You may be able to smell faint whiffs of ammonium (this is fine) and the biscuits will expand dramatically. Remove from the oven and let them cool on a wire rack. Turn them over to check if the bases are light golden and firm to touch. These biscuits are inedible under-baked (the smell will linger), but over baked will make them tasteless. Trust your instincts!
