Combine warm water, granulated sugar, and yeast in a large bowl; let rest 5 minutes until yeast blooms.
Add half of the oil to the yeast mixture and set remaining oil aside.
Stir in flour and kosher salt until combined.
Place dough in a greased bowl, cover, and let rise in a warm place for 1 hour or until doubled.
Preheat oven to 450°F.
Pour remaining oil into a 9x11-inch baking pan and brush to coat bottom.
Transfer dough to the pan, spreading to edges.
Mix melted butter, brown sugar, cinnamon, and vanilla in a medium bowl until combined.
Fold diced apples into the butter mixture.
Spread one-third of the apple mixture over dough, then fold dough edges toward center over filling.
Flip dough seam side down and gently stretch back to edges.
Dimple dough surface with fingertips.
Spread remaining apple mixture evenly on top.
Bake 28-30 minutes until golden brown and apples are caramelized.
Whisk confectioners’ sugar with milk, adding milk 1 tablespoon at a time until glaze reaches desired consistency.
Drizzle glaze over warm focaccia before serving.