Use a food processor to pulse hazelnuts until finely ground. Combine the ground hazelnuts with melted butter, brown sugar, and egg white. Spray a tart pan with cooking spray and press the hazelnut mixture into the pan to create the crust. Bake for 15-18 minutes or until golden brown. Cool the crust completely.
Reduce the oven temperature to 325 degrees F.
Make the tart filling. Melt butter with ⅓ cup sugar in a small pot over medium-low heat until the sugar has dissolved. Pour the butter over the bittersweet chocolate in a large bowl, stirring until the chocolate has melted.
Beat together the remaining ⅓ cup sugar with 3 eggs using a whisk attachment until it's pale yellow and thickened, about 3-4 minutes. Add ⅓ of the egg mixture to the chocolate, stirring until well combined. Fold in the remaining egg mixture to the chocolate, adding ⅓ of the eggs at a time. Pour the chocolate filling into the hazelnut crust.
Peel and thinly slice 3 pears about ⅛ inch thick. Arrange the sliced pears on the chocolate filling. Combine cinnamon with granulated sugar and sprinkle the cinnamon sugar on top of the pears. Arrange hazelnuts on top.
Bake the tart for 40-45 minutes or until the edges have set. The center of the tart will still be a little wobbly. Remove from the oven and cool completely.
Notes
The pear chocolate hazelnut tart should keep for 5 days if stored in an airtight container.