Scald the buttermilk (heat just until steam rises), then sprinkle the yeast on top and gently stir, set aside to proof while you prepare the rest of the ingredients, 5-10 minutes.
In a medium bowl, whisk together the flour, sugars, salt, powdered vanilla beans, and cardamom to combine. Combine the yogurt, eggs, orange blossom water, vanilla, butter and yeast mixture in a large bowl and stir to combine. Add half of the flour mixture, stir with a wooden spoon, then add the second half and stir until no dry bits remain and the flour is incorporated. Cover with a clean kitchen towel and allow to rise in a warm place for 1-1/2 hours. Prepare 2 round 8-9 inch baking pans with a square of parchment paper to prevent sticking and make the filling.
To prepare the filling, mix all the ingredients together in a small bowl.
Once the dough has risen. lightly dust a clean work surface with flour and place an additional ½ cup of flour nearby, for adding to the dough when needed. Punch the dough down and scrape out onto your work surface. Knead, adding flour as needed, just until you have a workable dough. Pat and shape into a 9 x 13 rectangle. Spread your filling on the dough, leaving a 1 inch border. Sprinkle with a couple of pinches of flake salt, then scatter 1 cup of the rhubarb on top. Roll, starting with one long side and rolling to the other long side, pinching the dough together with your fingers to seal. Pat into an even log shape and cut into 10 even rolls using dental floss or fishing wire, reshape if needed and place in the baking pans. Space them evenly if you like your rolls to have firmer edges, or fit as many as you can (7 or so) into one, and the remaining in the second pan if you like more doughy rolls (what I did). At this point you can refrigerate the rolls overnight or proof and bake.
Cover the rolls and allow to rise for 45 minutes, 60 if they've been in the fridge. About halfway through the second rise, preheat the oven to 350 degrees.
Brush the rolls with a little milk or cream, then sprinkle with the remaining rhubarb and bake for 25 minutes, then check for doneness. The rolls should be lightly brown and just cooked, try not to overbake (mine took around 27 minutes). To make the glaze, combine all the ingredients in a bowl and mix to combine, then spoon over the warm rolls and serve.
Recipe by oh honey bakes at https://ohhoneybakes.com/soft-buttermilk-cardamom-rolls-rhubarb/