Preheat oven to 350°. Line a large rimmed baking pan with parchment paper. Spread hazelnuts in another (unlined) baking pan and bake just until beginning to turn golden under skins, 5 to 7 minutes. Pour into a clean kitchen towel and rub to remove as many skins as possible.
In a large ovenproof frying pan, mix hazelnuts with superfine sugar. Bake until sugar is lightly browned, about 20 minutes. Remove from oven and stir immediately. Place over medium-high heat on the stove and cook, stirring constantly with a wooden spoon, until sugar is melted and golden brown, about 2 minutes. Pour immediately into the lined pan and separate hazelnuts gently with spoon. Let cool.
Meanwhile, peel pears, leaving them whole and with stems intact. With the large end of a melon baller, scoop bottoms of cores from pears; then with the smaller end, dig deeper and scoop out seeds. Cut a thin slice off bottoms so pears stand flat.
Preheat oven to 400°. Melt butter in a large ovenproof frying pan over medium-high heat. When it starts to brown, add pears and cook, gently turning occasionally with 2 wooden spoons, until golden brown all over, 5 to 7 minutes. Sprinkle pears with granulated sugar and continue cooking and turning pears until sugar is caramelized and thick, about 5 minutes.
Remove pan from heat and add the brandy. Return to heat and boil until liquid is reduced to a thick, bubbly syrup, 10 to 12 minutes. Pour in 1/2 cup very hot water and stand pears upright in pan. Bring liquid to a boil; transfer pan to oven.
Bake, basting every 15 minutes, until pears are tender when pierced and liquid has thickened again and is dark gold, 25 to 30 minutes. If liquid reduces too quickly, add more water, 1/4 cup at a time. Let pears cool slightly in liquid, basting often.
Transfer pears to shallow bowls. Stir lemon juice into syrup, then spoon over pears. Add a scoop of chocolate sorbet to each bowl and sprinkle with toasted hazelnuts.
Make ahead: You can toast the hazelnuts up to 3 days ahead; store airtight at room temperature. You can also bake the pears (through step 6) up to 4 hours ahead; let stand at room temperature. Rewarm over medium heat, basting pears with syrup, about 3 minutes.