Put the raisins in a medium sized bowl and pour enough boiling water to cover them. Let it sit for ten minutes. After that time, drain the raisins, squeeze them to release the extra moisture and set aside.
In a large bowl, combine the rye and whole wheat flours, the salt and toasted sesame seeds.
In another large bowl, dissolve the sourdough starter in the water. Add the flour mixture to the bowl with the(dissolved) starter and mix until a dough comes together. Cover the bowl with plastic wrap and let it sit in the fridge overnight.
The next day, take the dough out of the fridge and divide it into 12 portions. Put a reasonable amount of sesame seeds in a large plate. Shape each portion into a bun, brush the buns with a little water, and roll them into the sesame seeds.
Put the buns in a large tray lined with parchment paper, cover with a clean towel and let rise in a warm place for 1 ½ hours. In the meantime, pre heat the oven to 240c and place a baking tray in its lowest third. As soon as the oven reaches the right temperature, place the tray with the buns in the middle third of it. Add a few ice cubes to the baking tray in the lowest part of the oven and close the door. Bake for 25-30 minutes.
Cool the buns on a wire rack for at least one hour before eating.