If using active dry yeast, heat the milk in the microwave or on the stovetop until it is “fingervarm” (finger warm), meaning it should feel warm to the touch but not hot. Add your yeast, stir, and leave it for 5-10 minutes until the mixture starts to bubble up. If using instant yeast, simply add the yeast to the milk - there is no need to dissolve it.
Combine the sugar, egg, and egg yolks in a large bowl. Once the yeast mixture has bubbled up a bit, add it to the egg mixture, or if using instant yeast, add immediately. Mix well.
Then, add the salt, flour, and softened butter, and mix with a wooden spoon until the dough starts to come together. Tip it out onto a floured surface and knead until smooth and elastic (about 10 minutes). Add more flour if needed - the dough should be soft and smooth, not dry and not sticking to the surface. If using a stand mixer, allow it to knead the dough together for about 10 minutes. During the final 2 minutes of kneading, add the raisins to the dough.
Cover the dough with cling film or a clean dishtowel and place in the fridge overnight for 8 hours, or let rise at room temperature (if room temp is very cold wherever you are, put it in a warm place, i.e. under a light) for 1.5-2 hours or until doubled in size.
Once your dough has doubled in size, shape it into 12 balls and place on two baking sheets (or one if you want them to stick to each other) lined with parchment paper.
To shape the rolls properly, you should first divide your dough into equal sections. Take a piece, pinch together into a ball shape, then cup it in your hand with the top of the roll touching your palm. Place your hand on an unfloured surface with the seam of the roll touching the table. Move your cupped hand in quick, tight circles - this will cause the seam to come together and create tension on the surface of the dough, meaning it will rise upwards in the oven, instead of flattening.
Cover and let the rolls rise for another 30-45 minutes. In the meantime, preheat your oven to 400 degrees F (204 degrees C). To check if they are ready, use the poke test. Poke the dough with one finger. If the dough springs back immediately, it needs more time. If it springs back slightly but an indentation remains, it's ready to be baked.
Brush with egg wash (whisk together one egg and some water until there are no more gloopy egg streaks). Bake for 10-12 minutes. Check the bottom of the buns to see whether they are done – they should be golden brown. If they’re too pale, they’ll need another minute or two.