1. To make the dough, place the warm milk in a bowl with the sugar and yeast. Allow to stand for 5 minutes, before adding the butter, egg yolk and vanilla. Stir to combine. Place the flour and pinch of salt into a large bowl, pour in the milk mixture and, using a spatula, mix together until well combined.
2. Using a stand mixer fitted with a dough hook, knead the dough for 7-8 minutes, or until soft and sticky to the touch. Place the dough in a lightly oiled bowl and cover with kitchen film. Allow to stand at room temperature for 45 minutes to 1 hour, or until doubled in size. Using the palm of your hand press down on the surface of the dough to release the gases formed during fermentation and shape into a ball. Place the dough back into the bowl, cover and place in the fridge to prove for 4 hours.
3. To make the cinnamon butter, place the butter, sugar and cinnamon into a bowl, and whisk until combined. Add the sultanas and mix. Set aside.
4. Lightly flour a work surface, and, using a rolling pin, roll the dough into a 20cm x 38cm rectangle to a thickness of approximately 0.5cm. Spread the cinnamon butter over the dough. Begin to roll inwards from the longest side of the dough. Once completely rolled, place in the fridge for 10 minutes.
5. Preheat the oven to 180°C, gas mark 4. Brush the inside of a 20cm round cake tin with a little melted butter and line the base with baking parchment.
6. Cut the dough into 9 pieces, then place into the prepared tin, allowing a little space between each piece. Leave in a warm place and prove for a further 20 minutes. Place on a baking tray and bake in the preheated oven for 25-30 minutes until golden. Remove from the oven. Allow to cool slightly before removing from the tin, and then drizzle with melted white chocolate and sprinkle with popping candy.