Whisk the brown sugar and 1/4 cup of the soy sauce in a bowl. Toss the pork in the mixture. Marinate in the fridge for 2 hours.
Mix together the hoisin, ginger, chili paste, garlic and remaining 2 tablespoons soy sauce in a small bowl. Set aside.
Heat a large skillet (12- to 14-inch) over high heat and add the oil. Toss the pork with the cornstarch to coat.
Add the pork to the skillet in small batches and cook, stirring infrequently to ensure good browning and crispness, until the pork is barely cooked through, about 5 minutes; remove with a slotted spoon and set aside on wire-racked baking sheet.
Add the sprouts, squash and scallions to the hot skillet and stir-fry until the veggies are crisp-tender and have developed good caramelization, 5 to 6 minutes. Add the pork back and cook 1 minute. Add the hoisin mixture and simmer for an additional 1 minute. Add the pears and toss to coat. Remove from the heat.
Very thinly slice the reserved scallion tops on the bias and toss them in with the sesame seeds to garnish.