Cinnamon-Date Sticky Buns

Combine buttermilk and 6 Tbsp. oil in a small microwave-safe bowl (it won’t get smooth and that’s okay) and heat in the microwave in three 10-second bursts until just about body temperature (it shouldn’t feel hot or cold to the touch); an instant-read thermometer will register about 98°F. (Alternatively, you can heat in a small saucepan over medium-low, stirring constantly, about 1 minute.)
Whisk egg, brown sugar, and yeast in a liquid measuring cup to combine, then whisk into buttermilk mixture.
Pulse flour, baking soda, and salt in a food processor to combine. With the motor running, stream in buttermilk mixture. Process until about 80% of the dough comes together into a ball (mixture will look very wet at first, then begin to pull away from the sides of processor), about 2 minutes.
Using a bowl scraper or rubber spatula, scrape dough onto an unfloured surface. (It should still be pretty wet and sticky.) Knead, pushing it away from you, then pulling it back toward you, until a smooth ball forms, about 3 minutes. The dough will grow silkier, tighter, and easier to work with as you knead.
Roll out dough into a rough 8" square. Fold dough over onto itself to make an 8x4" rectangle, then flatten it slightly and fold over once more to make a 4" square. Roll dough back out to an 8" square and repeat folding process (you should be back at a 4" square). Pour remaining 1 Tbsp. oil into a medium bowl and add dough (still folded); turn to coat. Cover bowl tightly and chill dough until doubled in volume, at least 8 hours and up to 1 day.
Place dates in a small bowl and pour in 1½ cups hot water to cover. Let soak until softened, about 10 minutes.
Drain dates and transfer to a food processor; discard soaking liquid. Add cinnamon, salt, and 2 Tbsp. oil. Purée, scraping down sides as needed, until smooth, about 4 minutes. (You should have about ½ cup purée.)
Grease a 10" cast-iron skillet with remaining 1 Tbsp. oil. Transfer dough to a clean, unfloured surface and roll out to an 8" square. Fold in half into an 8x4" rectangle, then fold rectangle over itself to form a 4" square (same folding process as you did before). If dough feels tough and uncooperative, let it sit about 5 minutes to relax and try again. Roll out dough to a 12" square about ¼" thick. Dollop date purée all over. Using a small offset spatula, spread evenly over dough, leaving a ½" border without purée along edge farthest from you. Sprinkle brown sugar over purée.
Starting at edge closest to you, roll up dough into a tight log. Using a sharp serrated knife and long sawing motions, trim about ½" of dough from both ends and discard. Slice log crosswise into 3 sections, wiping knife clean between slices. Slice each section crosswise into 3 buns (you should have 9 buns total that are each about 1" thick). Transfer buns to prepared pan as you go.
Cover pan tightly with foil (or a domed lid if you have one) and place in a warm, dry spot. Let buns rise until they’re doubled in volume and spring back when poked, leaving only a small indentation, 1–1½ hours, depending on how warm and humid your kitchen is.
Place a rack in middle of oven; preheat to 350°F. Bake buns, still covered, until puffed, pale, and mostly set, about 20 minutes. Remove foil and continue to bake until golden brown, about 15 minutes if you prefer a soft and squishy bun and up to 25 minutes for a more toasted bun. Let cool slightly.
Meanwhile, whisk powdered sugar, buttermilk, and vanilla in a medium bowl to combine.
Brush glaze over warm buns and serve in skillet.
Do Ahead: Purée can be made 3 days ahead. Place in an airtight container; cover and chill. Bring to room temperature before using.