In a large bowl, whisk together the all-purpose flour, potato flour, sugar, yeast, and salt. Add the softened butter, vanilla, egg, ricotta, and lukewarm milk, then stir until a shaggy dough forms. If time allows, let the dough rest for 30 minutes so the flour can fully absorb the liquid.

Knead the dough by hand, with a mixer, or in a bread machine on the dough cycle until it is smooth, soft, and just slightly sticky. Place the dough in a lightly greased bowl, turn to coat, cover, and let it rise until almost doubled in bulk, about 1 1/2 to 2 hours.

While the dough is rising, make the filling. For the cornstarch method, toss the grated apple with the lemon juice in a saucepan. Whisk the sugar, cornstarch, salt, and cinnamon together, then stir the mixture into the apples.

Cook the apple mixture over low heat, stirring often, until the apple begins to release its juice. Increase the heat to medium, bring to a simmer, and cook until thickened, about 1 to 2 minutes. Remove from the heat and let cool completely, or pop it in the refrigerator to speed things up.

If using Instant Clearjel, whisk the sugar, Clearjel, salt, and cinnamon together. Toss the grated apple with lemon juice, stir it into the sugar mixture, and set aside. No cooking is needed for this version.

Gently deflate the risen dough and turn it out onto a lightly floured or greased work surface. Divide the dough in half. Roll one half into a 10 by 12 inch rectangle, then spread half of the filling over the dough, leaving a 1/2 inch border. Sprinkle with half of the chopped nuts, if using.

Starting from a long side, roll the dough into a log and pinch the seam to seal. Use a sharp knife or bench scraper to cut the log in half lengthwise. Place the two pieces cut side up on a parchment-lined baking sheet and twist them together into a braid, working from the center out to the ends. Pinch the ends closed. Repeat with the remaining dough and filling.

Cover the twists loosely and let them rise for 1 to 2 hours, or until puffy. For an overnight make-ahead option, see the notes. Preheat the oven to 350 degrees F.

Bake the loaves for 30 to 40 minutes, until lightly browned. If they are browning too quickly after 20 minutes, tent them loosely with aluminum foil. An instant-read thermometer inserted into the bread, not the filling, should register about 190 degrees F. Let the loaves cool for about 1 hour before glazing.

In a small bowl, stir together the confectioners' sugar, vanilla, salt, and 1 to 2 tablespoons water or 2 to 4 tablespoons heavy cream until smooth. Drizzle the glaze over the cooled loaves. Store leftover bread loosely covered at room temperature for several days, or freeze unglazed loaves for up to a month.