Step 1: Preheat your oven to 400°F (200°C). Generously grease a 9x13 inch baking pan with 2 tablespoons of olive oil, ensuring the bottom and sides are well coated.
Step 2: In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, and 1 teaspoon of sea salt until well combined.
Step 3: Pour in the chilled sparkling water and the remaining 2 tablespoons of olive oil. Using a spatula or wooden spoon, mix until just combined. The dough will be shaggy and sticky – do not overmix! A few lumps are perfectly fine.
Step 4: Transfer the dough into the prepared baking pan. Using your oiled fingertips, gently press the dough evenly to the edges of the pan. Don't worry if it's a bit sticky; the oil will help.
Step 5: Drizzle a little extra olive oil over the top of the dough. Then, use your fingers to create characteristic dimples all over the surface of the focaccia.
Step 6: Sprinkle generously with flaky sea salt and your chosen optional toppings (e.g., fresh rosemary, halved cherry tomatoes, olives).
Step 7: Bake for 20-25 minutes, or until the focaccia is golden brown on top and cooked through. The internal temperature should reach around 200°F (93°C).
Step 8: Once baked, carefully remove the pan from the oven. Let the focaccia cool in the pan for a few minutes before transferring it to a wire rack to cool slightly more.
Step 9: Slice and serve warm. This focaccia is best enjoyed fresh!