Prepare the Dough (Night Before Baking): In a large mixing bowl, combine the 100g active sourdough starter, 375g filtered water, 30g honey, until milky. Then add 500g unbleached bread flour, and 10g salt. Mix until fully incorporated.
Perform a couple of sets of stretch and folds. Cover and let the dough Bulk ferment at room temperature overnight (8–12 hours). This step is flexible, and even over fermented dough will work!
The next morning: Preheat your oven to 425°F (218°C). Peel and chop 1 apple then set aside. In a small bowl, melt the 1 stick of unsalted butter and use a portion to grease a 9x13” baking pan or 12" cast iron skillet. Mix the remaining melted butter with 1- 2 tbsp. of cinnamon and 1/4-1/2 cup brown sugar.
Transfer the dough into the greased baking pan, gently stretch it to the sides of the pan so its covering the bottom. Pour the cinnamon sugar butter over the dough. Evenly distribute the sliced apples on top. Use your fingers to dimple the dough, pressing the toppings into it.
Bake: Cover the pan with foil and bake at 425°F (218°C) for 15 minutes. Remove the foil and bake uncovered for an additional 20 minutes, or until golden brown.
Prepare the Glaze: While the focaccia bakes, whisk together 1 cup powdered sugar, 1 tsp vanilla extract, 1 tbsp. milk, and a pinch of salt until a smooth icing forms.
Finish & Serve: Remove the focaccia from the oven, check internal temp (205°F), and let it cool a few minutes, and drizzle the glaze over the top.