Begin by making the tangzhong, place the milk and bread flour in a saucepan and whisk it gently until it thickens and reaches 65 degrees.
Scrape the tangzhong into a bowl and cover it with a layer of clingfilm so that a skin doesn't form. Leave to cool to room temperature.
Put the milk, butter and vanilla into a small saucepan and cook over a low-medium heat until all the butter has melted. Set aside until just lukewarm.
To make the dough, put the flour, caster sugar, salt and yeast into the bowl of a food mixer and fit it with a dough hook.
Make a well in the middle of the flour mixture, then pour in the milk mixture, the tangzhong and the egg.
Mix the dough on a low speed until it starts to come together, then knead on a medium speed for about 10 minutes or until the dough comes away from the side of the bowl.
Place the dough in a large lightly greased bowl and cover with cling film. Leave it to rise for about 1-1½ hours or until it has doubled in size.
Tip the dough onto a lightly floured work surface and use a rolling pin to roll into approximately a 40cm x 50cm rectangle.
Take 300g of the mincemeat and stir it to loosen a little, spread the mincemeat over the dough and spread it in an even layer. If you think you need more mincemeat then use the rest to fill in any gaps, but this can be quite rich, so use the smaller amount if you're not a mincemeat fiend.
Roll up the dough into a cylinder along the long edge, keeping the spiral tight. Using a sharp knife, cut into 12 even slices.
Arrange the buns in the base of a 20cm x 30cm non-stick baking tin, with the spiral facing upwards. Cover the tin loosely with cling film, set aside and allow the buns to prove for 45-60 minutes until risen and touching each other.
While the buns are proving, heat the oven to 180°C/160°C fan/gas 4. Once proved, bake the buns for 25-30 minutes or until golden brown. Remove the buns from the oven and allow to cool.
Sift the icing sugar into a bowl and add 2 tbsp of water and stir the icing, if needed add more water, but you want quite a thick consistency. Drizzle or pour the icing over the buns and use as much or as little as you want, you don't need to use all the icing. Tear off each bun as required and eat while still warm or at room temperature.