Heat oven to 325 degrees F. Coat an 8 x 8 x 2-inch baking pan with nonstick cooking spray.
In a large bowl, beat butter and sugar together until completely smooth. Next, beat in egg, vanilla extract and hazelnut liqueur. On low speed, stir in flour, hazelnuts and salt. Press evenly into bottom of prepared baking pan.
Bake at 325 degrees F for 35 to 40 minutes or until center is firm to the touch. Cool completely on a wire rack.
Place white chocolate in a small plastic food storage bag. Leave top open and microwave for 30 seconds. Press to soften. Snip off one corner of bag and drizzle chocolate over blondies. Cut into 16 squares.