In a large bowl, whisk together the flour, instant yeast, and salt until evenly combined.

Slowly add the warm water, stirring with a spatula until a sticky, wet dough forms. You should not knead — just mix until no dry flour remains.

Pour the dough into a lightly oiled 9×13-inch baking pan and spread it gently to fill the pan. Cover and let rise at room temperature for about 1–1½ hours, or refrigerate for up to 24 hours for a longer ferment and deeper flavor.

In a small saucepan over low heat, melt the butter and stir in the brown sugar and cinnamon until smooth. Pour this mixture over the risen dough, trying to distribute it evenly.

Use a spatula to fold the dough envelope-style a few times so the cinnamon-butter mixture gets incorporated into the dough, then smooth the top back out. Cover and let rest another 1–1½ hours until puffy.

About 30 minutes before baking, preheat the oven to 425°F (220°C).

Dimple the dough with your fingertips, drizzle any remaining cinnamon-butter mixture over the top, then sprinkle with the remaining brown sugar, cinnamon, and a pinch of flaky sea salt if using.

Bake for 20–25 minutes, until the focaccia is golden brown and caramelized around the edges.

Remove from the oven and let cool in the pan for 10 minutes before slicing and serving warm.