Heat oven to 325°F. Coat baking sheets with nonstick cooking spray or line sheets with nonstick aluminum foil. Fit large pastry bag with large 3/4-inch star tip.
In food processor, process hazelnuts and 1/4 cup sugar until finely ground. Add remaining sugar, the almond paste and egg whites. Process until thoroughly blended.
Spoon hazelnut mixture into pastry bag. Pipe rosettes, about 1-1/4 inches round, 1 inch apart, onto prepared sheets.
Bake in 325°F oven 14 minutes, until lightly golden. Transfer baking sheets to rack; let stand 1 minute. With metal spatula, remove macaroons to rack to cool. (If macaroons stand too long on baking sheet, they may stick. If so, place in oven 1 minute to help remove.)
Dip bottoms of macaroons in melted chocolate; place on waxed-paper-lined baking sheets. Let stand until set.