Preheat oven to 350°F (175°C). Grease and flour (or use a baking spray with flour) a 12-cup Bundt pan.
Whisk together the flour, baking powder, baking soda, cinnamon, and salt. Set aside.
Using an electric mixer on medium speed, beat the butter, brown sugar, and sugar until light and fluffy.
Add the eggs, one at a time, mixing well after each addition. Mix in the vanilla.
Reduce mixer speed to low. Add the flour mixture in 3 portions, alternating with 2 portions of sour cream. Mix just until combined.
Stir in the pears and hazelnuts.
Transfer the batter to the prepared pan. Bake 55 to 60 minutes, or until a pick inserted into the center comes out clean.
Cool the cake in the pan on a wire rack for 15 minutes. Then transfer the cake to a wire rack to cool completely.
To make the glaze:
Combine the confectioners' sugar, 2 tablespoons sour cream, and vanilla. Add more sour cream, a small amount at a time, until the glaze is thick but pourable.
Pour or spoon the glaze over the cooled cake. If desired, garnish with chopped hazelnuts.