Prep the pears: Toss sliced pears with melted butter, sugar, vanilla, and salt. Let them sit for 10 minutes to release their juices while you prep the ice cream base.
Make the ice cream base: In a saucepan over medium heat, warm heavy cream and pear cider until just simmering (about 185°F). Whisk egg yolks with sugar in a separate bowl until pale and slightly thickened. Slowly pour half of the hot cream mixture into the yolks while whisking constantly to temper them. Pour everything back into the saucepan and cook over low heat, stirring continuously, until the mixture coats the back of a spoon (about 170°F). Strain through a fine mesh sieve and chill in the refrigerator for at least 2 hours or overnight.
Churn the ice cream: Once chilled, churn according to your ice cream maker's instructions (or freeze in a shallow dish, stirring every 30 minutes until smooth). Set aside.
Prepare the soufflé dish: Butter and sugar-coat two 10-ounce ramekins or one large soufflé dish. Set aside.
Make the filling: Whisk flour into the milk until smooth, then stir in the pears with their juices. Bring to a gentle simmer over medium heat, stirring frequently, until thickened (about 5 minutes). Remove from heat and let cool slightly.
Beat the egg whites: In a clean bowl, beat egg whites until stiff peaks form. Fold them gently into the warm pear mixture in two additions — first half, then remaining half, being careful not to deflate too much of the air.
Fill and bake: Divide filling between prepared ramekins or pour into one large dish. Bake at 375°F for 20 minutes (ramekins) or 35-40 minutes (large dish). The soufflé is done when it's puffed about an inch above the rim and a skewer inserted in the center comes out clean.
Serve immediately: Scoop cold pear cider ice cream alongside each portion while the soufflé is still warm. The contrast of hot filling with cold ice cream is what makes this dish special — don't let it sit too long or you'll lose that temperature difference.
