Make phyllo cups:
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Preheat oven to 350?F
Generously butter every other cup in 12-cup muffin pan
Place 1 phyllo rectangle on work surface; cover remaining phyllo with damp cloth to prevent drying
Brush rectangle with melted butter; sprinkle with 1/2 tablespoon sugar
Place second phyllo rectangle atop first; brush with butter and sprinkle with 1/2 tablespoon sugar
Repeat 3 more times; top with 1 more rectangle and brush with butter, making stack of 6 rectangles
Using small sharp knife and 6-inch-diameter plate as guide, cut out two 6-inch round stacks
Press each stack into 1 buttered muffin cup
Repeat entire procedure 2 more times, making 4 more phyllo cups (6 total)
Bake cups until golden and crisp, about 15 minutes
Carefully lift cups, twisting slightly to loosen; place on rack
Cool completely
(Can be made 2 days ahead
Store airtight at room temperature
)
Make pastry cream:
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Whisk 1/2 cup sugar and cornstarch in heavy medium saucepan to blend
Gradually whisk in milk, then yolks
Scrape in seeds from vanilla bean; add bean
Whisk over medium heat until pastry cream thickens and boils, about 6 minutes
Strain pastry cream into small bowl; press plastic wrap directly onto surface
Chill until cold, at least 3 hours and up to 2 days
Preheat broiler
Place phyllo cups on small baking sheet
Spoon 1/3 cup pastry cream into each cup
Overlap 5 or 6 banana slices atop pastry cream in each cup
Sprinkle bananas with additional sugar
Broil pastries until sugar topping caramelizes, turning sheet for even browning, about 2 minutes
Transfer cups to plates
Scoop sorbet alongside