Combine the pineapple juice, milk, instant yeast and brown sugar in the bowl of a stand mixer fitted with a dough hook*. Add the flour, sourdough discard, beaten egg, vanilla extract and salt. With the mixer on low, slowly pour in 2 Tablespoons of melted butter. Increase speed to medium and mix until a shaggy dough forms, about 2 minutes.
Transfer the dough to a smooth work surface and knead until smooth, about 3-4 minutes. The dough should be smooth, soft and just slightly tacky (not sticky). If needed, add more flour or milk (1 Tablespoon at a time) to reach your desired consistency.
Transfer the dough to a large oiled bowl and cover with plastic wrap. Let rise for 90 minutes, or until doubled in size.
Brush a 9x13-inch baking pan with the remaining 1 Tablespoon of melted butter and set aside.
Turn the dough onto a smooth work surface and divide into 15 equal-sized pieces. This doesn't have to be exact, but if you do want perfectly identical rolls, weigh the dough on a kitchen scale and divide that number by 15. For me, each roll weighs about 73 grams.Using your palm for friction, roll each piece of dough into a ball, then place the rolls into the prepared baking pan with the seam-side down. Repeat until you have 15 rolls.
Cover the baking pan with plastic wrap or a clean kitchen towel and let the rolls rise for 60 minutes until puffed and doubled in size.
Preheat the oven to 375℉. Lightly brush the rolls with egg wash. Bake for 20-25 minutes until golden brown. Let cool slightly in the pan, then brush with 1 Tablespoon melted butter before serving (optional).
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Notes
*This recipe can be made without a stand mixer. To do so, mix the dough in a large bowl with a wooden spoon, then knead by hand. To bake with active sourdough starter: This recipe can be made with active sourdough starter instead of sourdough discard. To do so, omit the commercial yeast and replace the sourdough discard with active starter (in this case, 200g active starter). You'll also need to adjust the rise times; I recommend using visual cues (i.e. the dough doubles in size, etc.)