Soft and Fluffy Chelsea Buns (Custard Raisin Buns)

BRIOCHE DOUGH

1. In the bowl of a stand mixer fitted with the dough hook attachment, combine the milk, yeast, sugar, flour, salt, eggs, and vanilla. 
2. Mix on medium speed for 6-7 minutes, until the dough has come together. 
3. Add the butter a little at a time, waiting until it is almost incorporated to add the next piece, until all of the butter has been added. Mix the dough for a further 6-7 minutes until smooth and elastic.
4. Lightly grease a medium bowl. Shape the dough into a ball and place in the bowl. Cover the bowl with plastic wrap and leave to stand on the counter for 20 minutes, then transfer to the fridge to rise for at least 3 hours or up to overnight. (See recipe notes for doing this all in one day)

CUSTARD FILLING

1. In a medium bowl, whisk together the sugar and custard powder. Add the egg yolks and whisk until combined.
2. In a medium pot, warm the milk, vanilla bean paste and salt until there is movement just around the edges of the milk - do not bring it to the boil.
3. Remove the milk from the heat, and, whisking constantly, add half of the milk mixture into the egg and custard mixture to temper the egg yolks. Whisk briskly for 30 seconds. Transfer the milk-yolk mixture back to the pot, and return to a medium heat. Whisk constantly until very thick.
4. Remove from the heat and whisk in the butter, mixing well until totally combined.
5. Transfer to an airtight container and press a piece of plastic wrap directly against the surface. Refrigerate until completely chilled, ideally overnight. To do this quickly, you can spread it onto a sheet pan and press plastic directly against the surface.

ASSEMBLY AND BAKING

1. Lightly grease and line a 9" x 13" (23cmx35cm) baking dish.
2. Turn out the dough onto a lightly floured surface. Roll out to a 24" x 16" (60cm x 40cm) rectangle. If you find your dough is resisting being rolled out, stop and let it relax for about 5 minutes before continuing to roll. 
3. Stir the chilled custard. It may seem a little lumpy but will come together - this is just because it is thick. Using an offset spatula, spread the cooled custard evenly over the surface of the dough. Sprinkle with the brown sugar and raisins. Starting with the long edge, carefully roll up the dough tightly. Cut into 12 equally sized slices, and arrange in the baking dish. 
4. leave in a warm spot to rise until the rolls are puffy and when lightly pressed with a finger, an indentation that springs back slightly is left. This should take around 45 minutes to an hour, but will depend on the temperature of your kitchen. 
5. While the rolls are proofing, preheat the oven to 350°f / 180°c.
6. Bake the rolls for 30 to 35 minutes, or until the bread is cooked through and the top is golden. (internal temp 200˚f) Tent with foil after 20 minutes if the buns seem to be browning too quickly. Check for doneness after 30 minutes.
7. Remove from the oven and let stand for 10 minutes. While they are cooling, mix the sifted powdered sugar, vanilla, and 3 tablespoon of the milk in a medium bowl, adding extra milk a teaspoon at a time until you reach your desired consistency. Spread over the buns using an offset spatula while they are still warm. 
8. Serve warm or at room temperature. Leftovers are best eaten the day of, or the day after baking.