Add the dried blueberries to the hot water and allow them to soak for 30 minutes. This keeps the blueberries from absorbing the moisture from the bread.
While the blueberries soak, add warm milk and yeast to a small bowl and let it sit for 5 minutes to allow the yeast to bloom. Meanwhile, add the flour, sugar, eggs, and salt to the bowl of a stand mixer with the dough hook attached. Once the yeast has rested for 5 minutes, add the milk and yeast mixture to the stand mixer bowl.
Allow the dough to mix on a low speed until everything is combined. Once all of the flour has been absorbed, increase the speed to medium and add the butter 1 tbsp at a time. Once all of the butter is added and incorporated, drained the blueberries and pat them dry very thoroughly. Toss the blueberries in 1 tbsp of flour (around 10g). Add the blueberries to the dough as it mixes.
Let the dough mix for around 15 minutes, or until it has gained enough strength to pass the windowpane test. This means you can stretch the dough thin enough for light to pass through without the dough breaking.
Cover the dough and let it rise for 1.5-2 hours, until it has doubled in size.
Prepare the filling by adding the granulated sugar and lemon zest. Pinch it between your fingers to release the lemon oils. Then, add the butter and cream it together until smooth.
Once the dough is perfectly proofed, roll the brioche dough on a floured surface into a large rectangle, roughly 24”x20”. Evenly spread the filling over the dough, then fold it in half to create a 12”x20” rectangle.
Slice the rectangle with a pizza cutter lengthwise to create 8 long strips, each 20” long. To shape the buns, wrap each strip around your fingers 3-4 times, then cross over the center and securely tuck underneath. Place each shaped bun on a parchment lined baking sheet.
Cover the shaped buns with plastic wrap and allow them to complete a second rise for 1-2 hours. The buns should be noticeably larger, light, and airy after the second rise.
Preheat the oven to 400°F. Brush the proofed buns with egg wash and bake for 20-25 minutes, until the buns are perfectly golden brown and reach an internal temperature of 210°F.
While the buns bake, prepare the glaze by whisking together the powdered sugar, lemon juice, and vanilla bean paste. Let the buns cool before glazing.