Paste for crosses
Grease a 6cm-deep, 23cm (base) square cake pan. Place yeast, milk and 1 tablespoon sugar in a bowl. Whisk to dissolve yeast. Cover. Set aside in a warm place for 10 minutes or until foamy.
Sift flour, cocoa and mixed spice into a bowl. Rub in butter until mixture resembles fine breadcrumbs. Make a well in the centre. Add yeast mixture, egg and remaining sugar. Stir to combine. Turn dough onto a lightly floured surface. Knead for 1 minute. Transfer to a lightly oiled bowl. Cover. Set aside in a warm place for 1 hour or until doubled in size.
Punch dough down. Turn onto a floured surface. Knead for 5 minutes or until smooth. Add the combined choc chips. Knead to combine. Roll into 16 equal-sized balls. Place in prepared pan. Cover. Set aside in a warm place for 30 minutes or until dough has almost doubled in size.
Meanwhile, preheat the oven to 200C/180C fan-forced.
Make Paste for crosses: Combine flour, sugar and 2 tablespoons of cold water in a bowl. Spoon the paste into a snap-lock bag and seal the bag. Snip 1 corner from bag. Pipe crosses onto the top of each bun. Bake for 10 minutes. Reduce heat to 180°C/160°C fan-forced. Cook for a further 20 minutes or until the buns are golden-brown and cooked through.
Make Glaze: Sprinkle gelatine and sugar over boiling water in a heatproof jug. Stir with a fork until gelatine has dissolved. Turn buns, top-side up, onto a wire rack. Brush tops with gelatine mixture. Cool. Serve.