For crust: Using electric mixer, beat butter and sugar in bowl until fluffy. Mix in yolk. Combine flour, pecans and nutmeg; add to butter mixture and mix just until dough forms. Shape dough into ball and flatten to disk. Wrap in plastic and chill at least 3 hours. (Can be made 1 day ahead. Soften slightly before rolling out.) For filling: Whisk yolks and 3/4 cup sugar in bowl. Bring cream to simmer in small saucepan. Slowly whisk cream into egg mixture. Whisk in pumpkin and spices. Cool 1 hour. Roll out dough to 1/2 inch. Transfer to 10-inch-diameter tart pan with removable bottom. Press dough evenly to 1/4-inch-thick layer (dough is fragile). Press broken edges together. Freeze 15 minutes. Line with foil and fill with pie weights. Bake at 350 F until set, about 25 minutes. Remove foil and bake until golden, about 10 minutes more. Pour filing into crust. Bake until set, about 25 minutes. Transfer to rack and cool. Chill until cold, 2 hours or overnight. Preheat broiler. Sprinkle 1/4 cup brown sugar evenly over top of tart. Broil tart about 6 inches from heat, until sugar melts and caramelizes, watching closely to avoid burning tart and turning if necessary for even cooking, about 30 seconds. Serve immediately with lightly sweetened whipped cream, if desired. Serves 8.