In a large bowl, mix together the water, sugar, and yeast. Allow the mixture to rest for about 5 minutes, until the yeast starts to bloom and becomes frothy.

Pour half of the vegetable oil into the yeast mixture. Set aside the remaining oil for later use.

Gradually add the all-purpose flour and kosher salt to the yeast mixture. Stir gently until everything is well combined.

Place the dough into a greased bowl and cover it with a clean cloth. Allow it to rise in a warm location for about 1 hour, or until it has doubled in size.

While the dough is rising, preheat your oven to 450°F (232°C) to get it ready for baking.

Pour the remaining vegetable oil into a 9×11-inch baking pan. Use a pastry brush to spread the oil evenly across the bottom of the pan.

Once the dough has risen, transfer it to the prepared baking pan, spreading it out gently to reach the edges.

In a medium bowl, combine the melted butter, brown sugar, ground cinnamon, and vanilla extract. Stir until everything is fully blended.

Fold in the diced Granny Smith apples into the butter mixture, ensuring they are well coated for even flavor distribution.

Spoon ⅓ of the apple mixture over the dough. Gently fold the edges of the dough towards the center over the filling, creating a cozy pocket.

Carefully flip the dough over so the seam side is facing down. Gently press and stretch the dough back out to the edges of the baking pan.

Use your fingertips to create deep dimples across the surface of the dough, allowing for a beautiful texture.

Spread the remaining apple mixture evenly on top of the dough, allowing those lovely flavors to infuse while baking.

Place the pan in the preheated oven and bake for 28-30 minutes, or until the focaccia is golden brown and the apples appear bubbly and caramelized.

While the focaccia is baking, whisk together the confectioners' sugar and milk in a small bowl, adding milk one tablespoon at a time until the glaze reaches your desired consistency.

Once the focaccia is out of the oven, drizzle the prepared glaze over it while it’s still warm, creating an irresistible shine.