Sift the flour, spices and bicarbonate of soda into a large bowl and add the butter, cut into small chunks. Gently rub together with your fingertips - or pulse in a food processor - until the mixture resembles fine breadcrumbs
Add the egg yolk, sugar, syrup, treacle and zest and mix together until you have a firm dough. If it’s too sticky, mix in a little more flour. Wrap in clingfilm and chill for at least 1 hour.
Remove from the oven and leave to firm up a little for a few minutes, then gently transfer to a wire rack to cool. They will become crisp.