Whisk together the buttermilk and sugar in a heat-safe glass measuring cup (you can also do this in a small pan on the stove). Microwave in 15-20 second increments until the temperature reads between 100-110°F on an instant-read thermometer – you do not want it any hotter than 115°F or it will kill the yeast.
    Sprinkle the yeast over the surface of the milk and allow it to sit until foamy, about 5 minutes.
    Pour the mixture into the bowl of a stand mixer and then add in the egg, egg yolk, and melted butter. Whisk to combine.
    Stir in the flour, salt, and lemon zest and mix with the dough hook until a thick, shaggy, and somewhat sticky dough forms (alternatively, this can be done by hand).
    Lightly flour your countertop and turn the dough out from the bowl. Knead the dough for a few minutes with your hands until it is elastic and smooth.
    Place the kneaded dough ball into a greased bowl.
    Cover the bowl and set in a warm place to rise for 1 hour or until it has doubled in size.

    Roll the dough out to 1/4-inch thickness on a well-floured surface; you want a large rectangular shape roughly 13 x 16 inches.
    Spread the cinnamon-butter mixture evenly over the surface of the dough.
    Spread the blueberry filling evenly over the butter mixture.
    Gently roll the dough into a log, working from the longest end of the rectangle. Pinch the seam closed with your fingers.
    Using a knife or a piece of dental floss, cut the log into 12 even pieces and place cut-side up in a buttered 9 x 13-inch baking dish.
    For the Final Rise & Baking:
    Cover the baking dish with a towel and set aside to rise again for 20 minutes (at this point, you can also refrigerate the rolls overnight to use the next day. See recipe notes). Preheat the oven to 375°F during this time.
    After the rolls have risen the second time, transfer them to the oven and bake for 20-25 minutes or until puffed, golden, and bubbly in the centers.

    Generously glaze the rolls when they come out of the oven.