Heat the milk in a medium pot over low heat until it reaches 100 degrees F. Remove from heat and add in the yeast and 1/2 teaspoon sugar. Give it a quick stir. Let sit until foamy, which should take roughly 5-10 minutes. (If the mixture doesn't foam you may need new yeast or you could have killed the yeast with milk that was too hot. Start over or the rolls won't rise).
While the yeast is proofing, melt the 4 tablespoons butter and let it return to room temperature. Once cooled, whisk in the vanilla extract and egg yolk until smooth. Pour this mixture into the base of a stand mixer along with the remaining 1/3 cup sugar. Pour the activated yeast and milk mixture into the bowl of a stand mixer. Mix everything until combined. Add the flour and salt on top.
Mix on low speed with the dough hook until thick and slightly sticky (~2 minutes), scraping down the sides with a spatula as needed. Knead on medium speed until the dough gathers around the hook, (~5-6 minutes). If the dough is not gathering around the hook you can slowly add additional flour (1/4 cup up to 3/4 cup extra). You DO want the dough to be slightly sticky and tacky. Resist the urge to add too much additional flour as this will take away from the softness of the dough, but the dough should gather around the dough hook and be slightly sticky and tacky, but not stick to your fingers.
Remove the dough and shape it into a ball (again, it will be a little sticky!). Grease the mixing bowl with cooking spray or butter and return the dough to the bowl, turning to coat with the spray or butter. Cover with plastic wrap or a towel and let rise until doubled. This takes roughly 45 mins to 1 hour 15 minutes. Placing the dough in a warm(but not too warm)environment will make it rise a bit faster.
Lightly flour a clean surface and roll out the dough. Try to get a rectangle shape that is approximately 15 x 12 inches (with the 15 inch side facing you). Avoid overworking the dough.
For the Filling:
With a table knife, spread dough evenly with 4 tablespoons of softened butter (leaving a 1- inch border on the dough). In a small bowl, combine the sugar and lemon zest. Rub together with your fingers until fragrant and zest is broken down. Add the lemon juice and mix. Sprinkle this lemon sugar over the butter. Sprinkle and gently press the blueberries evenly over the lemon sugar mixture.
Using a very sharp knife (I use a bread knife), cut the roll of dough into 12 equal slices and place (cut side up) in a well-greased 9x13-inch ceramic or glass pan (avoid metal pans if possible; metal pans will sometimes make the sweet rolls taste metallic and react with the acid in the recipe).
Preheat the oven to 375 degrees F. Cover the rolls in the pan and allow them to rise until nearly doubled, about 20-30 minutes.
Warm the heavy cream for 15-20 seconds in the microwave (you don't want it hot, just not cold/straight from the fridge).
Using a pastry brush, brush the heavy cream over the risen rolls and allow it to soak down in and around the rolls.
Bake cinnamon rolls at 375 degrees F for 30-35 minutes, until the rolls are lightly golden brown at the edges and the center of the rolls are cooked through.
For the Frosting/Glaze:
While the rolls are cooling, prepare the glaze. Whisk together the lemon zest, lemon juice, melted butter, powdered sugar, and salt.
Allow sweet rolls to cool at room temperature for 5-10 minutes and then spread the glaze evenly over the rolls. Enjoy warm!