Put one tablespoon of sourdough starter into a clean container or pint size jar. Feed it with 60g water and 60g white flour. Stir until no dry flour is left. Let it sit at room temp 6-12 hours until active (doubled in volume, full of large bubbles, and smells fragrant like ripe fruit)

Scald the milk: Add more whole milk than the recipe calls for to allow for evaporation. Heat the milk over medium to medium-high heat until it is steaming and proteins begin to stick to the pot. Temperature will be 130 to 140 degrees F. Don't let it foam. Transfer 100g of your scalded milk to the bowl of a stand mixer, large bowl, or a separate glass carafe to cool down to 90-100 degrees F.

Knead your dough into a smooth, soft ball. It will take 5 to 8 minutes to get combined and soft until it pulls away cleanly from the sides of the bowl. Kneading by hand will take 8 to 12 minutes.

Now, it is time to knead in the softened unsalted butter. Add it one tablespoon at a time with kneading in between each addition. Continue kneading until the butter is completely incorporated into your dough and it pulls away cleanly from the sides of the bowl or kneading surface and is a soft smooth ball.

Place your cinnamon roll dough into an airtight container or use a large clean bowl covered with plastic wrap. Keep your dough at room temperature for 4 to 8 hours for the bulk fermentation or first rise until it doubles.

Prepare a 10 to 12-inch cast-iron skillet. You can substitute a large pie dish if you don't have a skillet. Rub 1 tablespoon butter on the inside of the skillet, covering the bottom and up the edges of the pan. Sprinkle a light coating of flour over the butter.

Prepare the filling: Peel apple then cut into thin strips that are 1/16 to 1/8 inch thick and about 1-inch wide. In a separate bowl, mix sugar and cinnamon together.

To shape the dough, lightly flour a work surface. Place dough on work surface. Dust flour onto a rolling pin and roll dough until it is a 12-inch by 12-inch square that is 1⁄2-inch thick.

Next, take the softened butter for the filling and rub onto the rolled out dough by hand until it is completely covered. Sprinkle the cinnamon sugar mixture onto the dough evenly until it is gone.

Use a pizza cutter or bread knife to cut the dough vertically into 12 1-inch wide strips. Starting with the center strips (the outside edges are usually smaller and should be saved for last), place strip cut-side down in the pan and curl strip into a loose spiral with the cinnamon sugar coated side facing in, in the center of the pan. There should be about 1⁄4-inch of room left between the spiraled strip layers for the dough to rise and for the apples to fit. Take the next strip, place it in the pan with the cut-side down and cinnamon sugar coated side facing toward the dough spiral that’s forming. Pinch the ends together and continue the spiral.

Continue with remaining strips of dough. When final strip is placed in pan, pinch the end of the strip to the outside of the spiral to close the pattern. Then, take apple slices and start at center of spiral and place apples in between every layer until it is filled.

Cover the sourdough apple cinnamon bread with plastic wrap. Let the covered dough sit at room temperature for the proof or second rise for 1.5 to 4 hours. The spiraled apple cinnamon bread should rise and spread until 1.5 to 2 times larger. The dough should feel soft to touch.

Preheat oven to 375 degrees F. Make sure to use regular bake, not convection.

Place pan into oven and bake 30 to 40 minutes, check pan after 20 minutes and tent with aluminum foil if the top is browning too quickly. The bread should have a golden brown top and the internal temperature of the bread should be 190-195 degrees F.

Cool 20 minutes before adding the vanilla glaze topping.

As the bread is cooling, prepare the vanilla glaze icing. Whisk together vanilla icing ingredients until it is a drizzling consistency. Use a spoon to drizzle the icing over the bread. I like to create an alternating zigzag or spiral pattern.

Cut the skillet sourdough apple cinnamon bread into pie-shaped pieces and serve!