Heat oven to 350°F. Line 4 cookie sheets with cooking parchment paper.
In 1-quart saucepan, melt butter over medium heat. Add brown sugar, granulated sugar and corn syrup; cook 2 minutes, stirring constantly, until sugar is dissolved. Remove from heat. Stir in hazelnuts, flour and vanilla.
Quickly spoon batter by heaping teaspoonfuls about 3 inches apart onto cookie sheets. Spread rounds to 1/4-inch thickness using back of spoon.
Bake 10 to 12 minutes or until dark golden brown. Immediately slide parchment paper with cookies onto cooling racks. Cool completely, about 30 minutes.
For each sandwich cookie, spread 1 heaping teaspoonful hazelnut spread on bottom of 1 cookie. Top with second cookie, bottom side down; gently press together. Let stand until set, about 1 hour.
In small resealable freezer plastic bag, place white chocolate; seal bag. Microwave on Medium (50%) 1 minute. Turn bag over; microwave 15 seconds longer until chocolate is softened. Gently squeeze bag until smooth. Cut off tiny corner of bag. Drizzle melted chocolate in 6 rings on top of each sandwich cookie. Starting at center, pull toothpick through rings to look like spiderweb. Let stand until set, about 30 minutes.