Step 1Grease a 4cm-deep, 18cm x 28cm (base) slice pan. Place yeast, sugar and milk in a bowl. Stir until combined and yeast is dissolved. Cover and set aside in a warm place for 10 minutes or until foamy.
Step 2Sift flour and mixed spice into a bowl. Rub in butter until mixture resembles fine breadcrumbs. Make a well in the centre of flour mixture. Add yeast mixture, 1/4 cup warm water, egg and sultanas. Stir to combine. Cover and set aside in a warm place for 1 hour or until doubled in size.
Step 3Preheat oven to 200°C/180°C fan-forced. Turn dough onto a floured surface. Knead for 5 to 8 minutes or until smooth. Form into 12 balls. Place balls in prepared pan, 1cm apart. Cover and set aside in a warm place for 45 minutes or until balls double in size.
Step 4Make flour paste: Whisk flour, sugar and 1/4 cup cold water together in a jug. Spoon into a snap-lock bag. Snip off 1 corner from bag. Pipe crosses onto buns. Bake buns for 10 minutes. Reduce oven temperature to 180°C/160°C fan-forced. Bake for 15 minutes or until golden and cooked through.
Step 5Place 3 teaspoons hot water in a jug. Sprinkle gelatine over water. Stir with a fork until gelatine dissolves. Turn buns onto a wire rack. Brush tops with gelatine mixture. Allow to cool. Serve.