Prep the Dough Liquids: Preheat oven to 170°F (76°C) for 10 mins, then turn it OFF. In a small saucepan, combine the cream, water, ½ cup butter, brown sugar, vanilla, and salt. Simmer on low just until the butter melts. Cool for 5 minutes.
Knead and First Proof: In a stand mixer (paddle attachment), combine half the flour, yeast, and ½ Tbsp lemon zest. Slowly pour in the butter mixture on low. Add eggs one at a time. Switch to the dough hook, add remaining flour, and knead on medium-low for exactly 10 minutes. Shape into a ball, place in an oiled bowl, cover, and put in the warmed, turned-off oven for 1 hour to rise.
Make the Compote: In a saucepan, boil the blueberries, lemon juice, brown sugar, salt, lemon zest, and ¼ cup butter. Reduce to medium and cook for 10 minutes. Whisk the cornstarch and water, pour it in, and stir constantly on medium-low until it resembles thick jam. Cool completely.
Roll and Fill: Brush a 9x13-inch baking dish with melted butter. Punch down the dough and roll it into a 20x14-inch rectangle. Spread the cooled bluberry compote evenly, leaving a 1-inch border at the top. Roll tightly from bottom to top, brush with melted butter, and seal the seam. Slice into 12 buns.
Second Proof and Bake: Place buns in the dish, brush tops with remaining melted butter, cover, and let rise for 30 minutes. Preheat oven to 350°F (177°C). Bake for 25-30 minutes until golden brown. Cool for 20 minutes.
Glaze: Whisk the powdered sugar, 2 Tbsp lemon juice, 2 Tbsp cream, and 1 tsp lemon zest. Pipe or drizzle heavily over the warm buns!