In a large mixing bowl, whisk together the sourdough starter and warm water until dissolved. Gradually add salt and flour until combined. Let rest for 30 minutes.

Perform a set of stretch and folds on the dough, then transfer to a greased bowl, cover, and let rise in a warm area for about 4 hours.

Preheat oven to 425°F and line a 9x13 inch pan with parchment paper. Coat with melted butter to prevent sticking.

In a saucepan, combine diced apples, butter, brown sugar, cinnamon, and vanilla over low heat. Cook for about 5 minutes. Reserve some filling and fold the rest into the dough.

Transfer dough to the pan, cover, and let rise for another hour until nearly double.

Create dimples on the dough surface and distribute the reserved apple mixture on top evenly.

Bake in preheated oven for 25-30 minutes until golden-brown and bubbly.

Cool for 10 minutes, then mix powdered sugar, milk, and vanilla to make glaze. Brush over warm focaccia.