Preheat oven to 350 degrees F. Spray a tart pan with cooking spray,
Combine hazelnuts and flour in a food processor and pulse until finely ground. Add melted butter, sugar, and egg white, mixing until well combined. Press the hazelnut mixture into the pan and up the sides to create the crust. Bake for 12-15 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 egg mixture at a time. Pour the chocolate filling into the hazelnut crust.
Peel and thinly slice 2-3 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.
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
You can bake the crust the day before and store it at room temperature until ready to use.Depending on the size of your pears, you may need 2 or 3 pears. I used 2 ½ pears for my tart.