Make the sourdough dough. In a large bowl, combine the bread flour, water, active sourdough starter, and salt. Mix with a wooden spoon, spatula, or your hands until no dry flour remains. The dough will be very sticky. Cover the bowl with a damp towel or plastic wrap.

Bulk ferment the dough. Let the dough sit at room temperature for 8 to 12 hours. The timing depends on your kitchen temperature. In a warmer kitchen (75 to 78°F), check around 8 hours. In a cooler kitchen (68 to 70°F), it may need the full 12 hours. The dough is ready when it’s puffy, nearly doubled in size, and full of visible bubbles. When you shake the bowl, the dough should jiggle.

Make the pesto. While the dough is fermenting, add the basil leaves, olive oil, parmesan, romano, butter, and garlic to a food processor or blender. Blend until completely smooth, stopping to scrape down the sides as needed. If making ahead, let the pesto come to room temperature before using (1 to 2 hours out of the fridge). It should be fluid and pourable, not thick with solidified butter. Reserve about ¼ cup of pesto for finishing.

Prepare the parmesan coating. Grate a block of parmesan on a box grater. For the finest coating, add the grated parmesan and cornstarch to a food processor and pulse until super fine and powdery. Transfer to a shallow bowl or plate and add the garlic powder if using. Whisk to combine evenly.

Set up your workspace. Line a 9x13 inch baking pan with parchment paper. Have your room temperature pesto, parmesan coating mixture, and lined pan all within reach.

Portion the dough. Once bulk fermentation is complete, lightly grease your hands with oil. Turn the dough out onto a lightly greased work surface or work directly from the bowl. Using greased hands or a bench scraper, tear the dough into 2 to 3 inch chunks. You should get about 20 to 24 pieces. They don’t need to be uniform.

Coat the chunks. Take each dough chunk and dip it into the pesto, turning to coat all sides generously. Let excess drip off briefly but keep them well saturated. Immediately roll the pesto coated chunk in the parmesan coating mixture, turning until all sides are coated. The pesto acts as glue for the coating to stick.

Arrange in the pan. Place each coated chunk into the prepared pan as you work. Nestle them close together but don’t pack them too tight. They should be touching or almost touching. This creates the pull apart effect as they proof and bake together.

Second proof. Loosely cover the pan with plastic wrap or a damp kitchen towel. Let it sit at room temperature for 1 to 2 hours. The chunks should puff up noticeably and look airy. They’ll expand and touch each other more firmly. Don’t over proof. The dough should be puffy but not completely doubled again.

Preheat the oven. While the dough is proofing, preheat your oven to 425°F (220°C). Make sure it’s fully preheated before baking.

Dimple the dough. Once the second proof is complete and right before baking, use your fingertips to dimple the entire surface. Press down gently all over, creating small indentations everywhere. This pushes the pesto and coating down into the dough, creating marbled swirls throughout. It also helps the focaccia maintain its texture during baking.

Bake. Uncover the pan and place it in the preheated oven. Bake for 20 minutes. The focaccia should be starting to turn golden brown.

Add the cheese. Pull the pan out of the oven and quickly sprinkle the shredded mozzarella evenly over the top if using. Work quickly to avoid losing too much heat. Return to the oven immediately.

Finish baking. Bake for another 5 to 10 minutes. The cheese should be fully melted and bubbly, and the focaccia should be deeply golden brown with some darker caramelized spots.

Add fresh pesto and serve. As soon as you remove the focaccia from the oven, drizzle the reserved fresh pesto all over the top. The heat will help it settle into the nooks and crannies while keeping the bright green color. Scatter fresh basil leaves over the top and sprinkle with flaky sea salt if desired. Let cool for about 5 minutes, then serve warm.