2-inch piece fresh ginger
1 cup sugar
3 cups warm water
1 vanilla bean, halved lengthwise
4 firm, barely ripe Bosc pears
1 or 2 tablespoons Poire William or other pear brandy (optional)
1 1/2 pints vanilla ice cream
Chocolate sauce or caramel sauce for serving
2 tablespoons chopped pistachios for garnish