Mix together ripe/active sourdough starter with all-purpose or bread flour and water. Cover loosely and let sit 10-12 hours at 78-80°F until doubled, bubbly & peaked.

To a large bowl, mix together ripe levain with water. Add salt, sugar and bread flour. Mix together using a dough whisk or spoon until a wet and sticky dough forms and the ingredients are fully combined. Cover and let rest for 30 minutes.

After the dough has rested, perform a series of 'stretch and folds' over the next 1 ½ hours. Wet your hand and stretch the dough up and over itself, turning the bowl a quarter turn each time. Cover and set aside.

After 30 minutes, perform your first set of stretch and folds. Cover the dough and let rest for 30 minutes.

Grate the cold butter. Put half of the grated butter on top of the dough. The other half should be placed in the refrigerator to chill until the next set of folds. Stretch and fold the dough again, incorporating the butter.

30 minutes later, add the rest of the butter to the top of the dough. Stretch and fold again. Cover and let rest for 30 minutes.

After 30 minutes do your final set of stretch and folds. Cover the dough and let rise for 6-7 more hours.

While the dough is resting, prepare the almond frangipane filling. Beat the softened butter until light and creamy. Add the sugar and continue beating until well combined. Mix in the egg yolk and almond extract, then add the almond flour, all-purpose flour, and salt. Beat until a thick, smooth paste forms.

Once the dough has risen about 70%, dump the dough out onto a counter and stretch it into a 14 by 16-inch rectangle. Add about ⅔ of the filling on top of the dough. Fold the dough over and then over again. Spread the remaining filling on the rectangle of dough. Roll the dough up and place it into a prepared banneton basket.

Cover the dough and refrigerate overnight or up to 12-18 hours.

Preheat a Dutch oven to 450°F for 20 minutes. Remove the sourdough loaf from the refrigerator. Dump it out on a piece of parchment paper and score your loaf.

Carefully place the loaf into the Dutch oven. Decrease the temperature to 425°F and bake for 25 minutes covered and 20 minutes uncovered. Let the bread cool on a baking rack.