In a small bowl, whisk together the cornstarch, 2 tablespoons of water, and vanilla extract until smooth. Set the slurry aside while the compote cooks.

In a medium saucepan over medium heat, combine the blueberries, sugar, lemon juice, lemon zest, salt, and butter. Cook, stirring occasionally, until the blueberries break down and release their juices, about 5 to 8 minutes. If using frozen blueberries, allow an extra minute or two as they release more liquid.

Pour in the cornstarch slurry and stir constantly for 1 to 2 minutes until the compote thickens. Remove from heat, let cool completely, then cover and refrigerate. The compote must be fully cooled and thick before dipping your dough balls.

In a large bowl, combine all of the dough ingredients at the same time: the bread flour, water, instant yeast, sugar, salt, and vanilla extract. Instant yeast does not need to be activated first and goes straight in with everything else.

Mix until no dry flour remains. The dough will look sticky and shaggy. This is completely normal for a high hydration focaccia dough.

Once the yeast is foamy, add it to a large bowl along with the 37 grams of flour and all remaining dough ingredients: the bread flour, water, sugar, salt, and vanilla extract. Mix until no dry flour remains.

Cover the bowl and let the dough rest for 30 minutes.

Remove the compote from the refrigerator and allow it to come to room temperature while you prepare your pan and coating.

Line a 9x13 inch metal baking pan with parchment paper and grease generously with butter or cooking spray.

In a shallow bowl or plate, whisk together the granulated sugar, cornstarch, and lemon zest for the coating. Set aside.

Dump the proofed dough out onto a greased surface. Pinch off a golf ball sized chunk of dough and, working with one piece at a time, dip it into the cooled blueberry compote and coat generously on all sides. Immediately roll it in the lemon sugar cornstarch mixture until fully coated, then place it in the prepared pan. Repeat until all the dough has been used, arranging the pieces so they are touching each other in the pan.

Cover the pan loosely with plastic wrap or a damp towel and let rise at room temperature for 1 to 2 hours, until the pieces have puffed up, grown together, and jiggle gently when you shake the pan.

Preheat your oven to 425 degrees F (220 degrees C). Once the dough has completed its second rise, dimple the surface by pressing your fingers firmly into the dough all over.

Bake for 20 minutes. Remove the pan from the oven and spoon any remaining blueberry compote generously over the top, spreading it into the crevices between the pieces.

Return the pan to the oven and bake for an additional 10 minutes, or until the internal temperature reaches 200 to 205 degrees F and the edges are deep golden brown. If the top is browning too quickly at any point, tent loosely with foil. Cool in the pan for 10 minutes.

While the focaccia cools, make the lemon glaze. Sift the powdered sugar into a bowl. Add the lemon juice one tablespoon at a time, whisking until smooth and thick. Add the optional heavy cream and whisk to combine. The glaze should drizzle slowly off a spoon.

Drizzle the lemon glaze generously over the warm focaccia, making sure every piece gets covered. Let the glaze set for a few minutes before serving. It will harden into a crackly finish as it cools.

Pull apart and enjoy.