Combine buckwheat and all-purpose flours in a mixing bowl. Add buttermilk and yeast. Stir with a spoon until batter is smooth. Add more buttermilk, one tablespoon at a time, if batter is too stiff.
Cover the bowl with a dinner plate and let stand in a draft-free place, 8 hours to overnight.
Add sugar, baking soda, and salt after the overnight rest. Mix in enough hot water to give batter a thin, pourable consistency.
Heat a lightly oiled griddle over medium heat.
Drop batter by large spoonfuls onto the griddle and cook until bubbles appear and edges are dry, 3 to 4 minutes. Flip and cook until browned on the other side, 2 to 3 minutes. Reserve some batter as a starter for the next batch.