Combine flour, oats, and brown sugar in a medium bowl. Cut in the butter and mix until crumbly. Press into the bottom of an ungreased 13 x 9-inch baking pan.
Bake for 15 minutes.
In the bowl of a standup mixer, beat together the granulated sugar, pumpkin, evaporated milk, eggs and spices. Beat at medium speed for 2 minutes until frothy. Pour over crust.
Bake for 25 minutes. While the bars are baking, toast the hazelnuts in a dry sauté pan over medium heat until they start to turn dark brown. Toss them around to ensure they are toasted on all sides. Chop hazelnuts in a small food processor. Combine chopped hazelnuts and brown sugar in small bowl. Cut in the butter and mix until crumbly. Sprinkle hazelnut topping over filling. Continue baking for 15 to 25 minutes or until knife inserted in center comes out clean. Cool completely in pan on wire rack. Cut into bars.
Recipe by StreamingGourmet at http://blog.streaminggourmet.com/2012/10/01/hazelnut-pumpkin-bars/