Blend flour, sugar, vanilla seeds, rind and 1 tsp sea salt in a food processor to combine, add butter, currants and candied lemon and process until a sandy texture. Add yolks, process to just combine, tip out onto a work surface and bring dough together with the heel of your hand. Form into a disc, roll out to 1cm thick on a piece of floured baking paper and refrigerate to rest (30 minutes).
Preheat oven to 180C. Cut out rounds with a 6cm cutter, place on a baking tray lined with baking paper and bake, turning tray occasionally, until golden around the edges (10-12 minutes). Set aside to cool on trays for 5 minutes.
For vanilla glaze, stir sugar, vanilla seeds and 50ml water in a saucepan over medium-high heat to dissolve sugar, then bring to the boil and cook until a light syrup forms (1-2 minutes). Remove from heat, add icing sugar, butter and 1-2 tbsp hot water to achieve a smooth glazing consistency.
Dip one side of warm shortbread biscuits into vanilla glaze, allowing excess to drip off, place on a tray lined with baking paper and stand until set (10-15 minutes). Store in an airtight container.