Mix the levain ingredients and briefly knead. Place the levain in a container with room for doubling, cover loosely, and keep warm, ideally 80-85F.
When the levain has doubled in size (about 6 hours in a lit oven) it is ready to use.
Put all the dough ingredients except for the butter in the bowl of a stand mixer with the dough hook attachment. Tear the levain into about 10 pieces when you add it to the bowl.
Run the mixer on low and then medium speed until the ingredients are incorporated.
Add the butter chunks one at a time while running the mixer, waiting about 30 seconds between each chunk.
Remove the dough from the mixer, working it between your hands to check for any unincorporated butter. If you find any butter chunks, smear and knead with your hands.
Place the dough in a bowl or straight-walled container, cover and keep warm (ideally 80-85F) until doubled in size (about four hours in a warm lit oven).
Prep the bun filling in three small bowls: one with the brown sugar, cinnamon, and sage whisked together, a second bowl with the raisins and bourbon or water, and a third bowl with cubed butter.
Lightly oil/butter your work surface and turn the doubled dough out onto it.
Gently pat and roll the dough out into a rectangle about 12"x18".
Brush on the melted butter, leaving about ½" uncovered on one of the long edges.
Sprinkle on the sugar-spice-herb mixture, gently smoothing it even with your palm.
Spread on the damp raisins.
Roll the dough into a tube, working toward the unbuttered long edge and letting the tube rest on the seam to seal up.
Grease 9" diameter cake pan. If not non-stick, then line the bottom with a round of parchment paper too.
Mark lines on the tube for eight buns and then cut the buns using a sharp serrated knife or dental floss looped around the tube.
Place the buns in the pan and cover loosely.
Let the buns rise until they almost fill the pan (about four hours if kept warm).
Preheat your oven to 180C
Bake the buns for 25-30 minutes, until the inside of the buns is about 190F.
Remove from the oven and cool for about 10 minutes.
Mix together the powdered sugar and milk for icing.
If serving after the 10-minute cooling period, flip the buns out of the pan and drizzle on the icing.
If serving later, run a flat knife around the inside of the pan to separate the buns from the edge, leave them in the pan to finish cooling, then wrap with plastic.
