In a large bowl, combine the water, active sourdough starter, salt, sugar, and vanilla extract. Mix until the starter has dissolved into the water.

Add the bread flour and mix until no dry flour remains. The dough will look sticky and shaggy at this point, which is completely normal.

Cover the bowl with a damp towel or plastic wrap and let the dough rest at room temperature for 1 hour.

After resting, perform one set of stretch and folds to build strength.

Cover the bowl and let the dough bulk ferment at room temperature for 8 to 12 hours, or until it has at least doubled in size.

In a small bowl, whisk together the cornstarch, 2 tablespoons of water, and vanilla extract until smooth.

In a medium saucepan over medium heat, combine the blueberries, sugar, lemon juice, lemon zest, salt, and butter. Cook until the blueberries break down and release their juices.

Pour in the cornstarch slurry and stir constantly until the compote thickens. Remove from heat, let cool slightly, then cover and refrigerate overnight.

Line a 9x13 inch metal baking pan with parchment paper and grease generously.

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

Dump the proofed dough out onto a greased surface. Pinch off a golf ball sized chunk of dough, dip it into the cooled blueberry compote, and coat generously.

Roll it in the lemon sugar cornstarch mixture until fully coated, then place it in the prepared pan.

Cover the pan loosely and let rise at room temperature for 1 to 2 hours.

Preheat your oven to 425°F (220°C). Once the dough has completed its second rise, dimple the surface.

Bake for 20 minutes. Remove the pan from the oven and spoon any remaining blueberry compote over the top.

Return the pan to the oven and bake for an additional 10 minutes, or until the internal temperature reaches 200 to 205°F.

While the focaccia cools, make the lemon glaze. Sift the powdered sugar into a bowl. Add the lemon zest and lemon juice one tablespoon at a time, whisking until smooth.

Drizzle the lemon glaze generously over the warm focaccia. Let the glaze set for a few minutes before serving.