Sourdough Focaccia | You Knead Sourdough

Ingredients

• 4 cups (512 g) all-purpose flour or bread flour
• 2 teaspoons (10 g) kosher salt
• 2 cups (455 g) sourdough starter, unfed or recently fed
• 1 1/2 cups (340 g) lukewarm water
• butter for greasing
• 4 tablespoons olive oil, divided
• flaky sea salt, such as Maldon
• 1 to 2 teaspoons whole rosemary leaves, optional