Mix the potash with the rum and lemon juice and set aside. Heat the butter, sugar and syrup until the mixture becomes glossy: it must NOT boil or overheat (no more than 65 degrees). Instead, it should melt very slowly whilst being stirred.
Once everything has melted, cool the pan in a bain-marie whilst stirring. Stir in the potash mixture. Sift the flour and spices together and gently stir them into the lukewarm mixture. Grate in the ginger, add the almonds, orange zest and the candied bitter orange/orange.
Finish kneading on the worktop, adding a little extra flour if necessary. Roll into logs (5 cm in diameter) and chill.
Cut into 2 mm-thick slices with a sharp knife and bake on baking trays at 170°C (fan oven) for 8–12 minutes. Make sure to turn the tray halfway through if the oven does not distribute the heat evenly, so that all the brownies are baked evenly. Cool on baking paper and leave to rest for a good hour before packing into airtight biscuit tins.
NB! Can be refrigerated until the next day; the dough can also be frozen.
