Make the dough: In a large mixing bowl, stir together the warm water, yeast, and granulated sugar. Let it sit for about 5 minutes until foamy. Add the sea salt, olive oil, and flour, mixing until a shaggy dough forms. Knead until smooth and slightly tacky.
First rise: Lightly oil a clean bowl, add the dough, and turn to coat. Cover and let rise for 1 hour, or until doubled in size.
Second rise: Line a baking sheet or skillet with parchment and drizzle with olive oil. Gently transfer the risen dough to the pan, stretching it toward the edges. Let it rise for 2 hours
Dimple the dough: Dimple with oiled fingertips, pressing deeply to form pockets.
Add the cinnamon mixture: Stir together the melted butter, brown sugar, cinnamon, and pinch of sea salt. Spoon the mixture over the dough, allowing it to settle into the dimples.
Bake: Bake at 400°F (205°C) for 20–25 minutes, or until golden around the edges and bubbling in the cinnamon pockets.
Glaze: Whisk the powdered sugar, milk, and vanilla until smooth. Drizzle over the warm focaccia and let it set before slicing.