Warm the milk to about 105F/40C. Whisk in the instant yeast and 20 g sugar, then stir in the 100 g bread flour until you get a smooth, thick paste.

Cover the bowl and let the starter rise at room temperature for 45 minutes, until bubbly and roughly doubled.

In a stand mixer bowl, combine the risen starter with the remaining 400 g flour, 80 g sugar, whole eggs, egg yolks, salt, lemon zest, and vanilla bean paste. Knead with the dough hook on low-medium for 10 minutes until the dough comes together and pulls from the sides of the bowl, still tacky.

With the mixer running, add the softened butter one cube at a time, waiting until each piece is fully absorbed before adding the next. Keep kneading for 8 to 10 minutes until the dough turns smooth, elastic, and shiny.

Shape the dough into a tight ball and place it in a lightly buttered bowl. Cover and let it rise at room temperature, about 24C/75F, for 2 to 3 hours, until doubled.

Butter the pandoro mold generously, working into every ridge. Punch down the dough, shape it into a tight ball again, and place it seam side down in the mold.

Cover the mold loosely and let the dough rise for 1.5 to 2 hours, until it sits about 1 cm below the rim. Don't let it rise above the rim, the star ridges blur once it bakes.

Heat the oven to 160C/320F. Bake the Pandoro for 35 to 40 minutes, tenting loosely with foil after 20 minutes if the top browns too fast, until the internal temperature reads 190F/88C and a skewer comes out clean.

Cool the loaf in the mold on a wire rack for 10 minutes, then invert and unmold. Let it cool completely, at least 2 hours, before doing anything else.

Once fully cool, dust generously with vanilla sugar through a fine-mesh sieve, turning the loaf slowly to coat every ridge. Slice into star-shaped wedges to serve.