Make the Starter: 8 to 12 hours before mixing your dough, combine the ingredients for the starter in a clean container. Cover and let activate until it doubles, is very bubbly, smells fragrant like ripe fruit.
Warm the milk: Measure out the milk (add a little extra to account for some of the volume to steam off) into a saucepan and cook over medium heat until you begin to see the steam rising off it or until it reaches a temperature of 130° to 140º F.
Scale: Place a mixing bowl on the scale, tare the weight of the scale, and pour in 180 grams of the warm milk. Add 28 grams of butter, the sugar or honey, and 200 grams of the starter, using a spoon to gently stir and break it apart. Add the flour. (When using the scale to measure ingredients, use the tare function to remove the weight of the mixing bowl and other previous ingredients so you can weigh each individual ingredient easily as you add each one to the bowl.)
Mix: Stir together with a spoon until partially combined. Add the egg yolk and salt and continue mixing the dough using a dough scraper to scrape the sides of the bowl. (if you have a stand mixer and a dough hook, use it to mix and knead the dough).
Knead: Knead the mixture with your hands and using a dough scraper, either in the bowl or on a clean work surface (or in the stand mixer) for 5 to 10 minutes, until the dough is smooth and releases easily from the bowl or work surface.
Bulk fermentation: Cover the dough and leave it at room temperature to ferment for 4 to 7 hours, until about doubled.
Stretch and folds: At least 30 minutes into the bulk fermentation, to further strengthen the dough, pull one quarter of the dough upwards and fold it over the middle. Repeat this process with the other three quarters of the dough. Re-cover the dough. This step can be done every 30 minutes, up to 3 times, during bulk fermentation for maximum development of dough strength.
Prepare the filling: In a mixing bowl, beat the butter, sugar, and cinnamon together to create a smooth, creamy mixture. Set aside.
Prepare the skillet: Line a large cast-iron skillet or a baking sheet with parchment paper.
Shape: Transfer the dough to a floured work surface. Using a floured rolling pin, roll the dough into a 12-by-18-inch rectangle. With a spatula, spread the filling over the dough with a spatula, leaving a ½ inch margin around the edges. Starting at one of the long sides of the dough, roll up the dough tightly into a long cylinder and press to seal the seam. Starting 2 inches from one end of the dough, cut the cylinder in half lengthwise, making sure the top is still connected. Starting where the dough is connected, braid the two open halves together so that the inside layers are exposed. Take the end of the braided dough and tuck it over the end that is connected and under where the braid began to create a round wreath shape. Carefully lift the wreath and place it onto the center of the parchment paper in the skillet or baking sheet.
Proof: Cover the dough with a kitchen towel and proof for 1.5 to 3 hours at room temperature, until nearly doubled and a finger lightly pressed into the dough slowly rises back.
Preheat: Preheat the oven to 375º F.
Bake: Bake for 30 to 35 minutes until it is golden-brown on the edges.
Cool: Transfer the skillet or baking sheet to a cooling rack and let cool for 10 to 15 minutes to allow the melted brown sugar and butter on the base to harden and create a sweet and crunchy bottom crust. Transfer to a plate and serve.